I've written about the tendency for progressive escalation of intensity in the D/s lifestyle before but if you haven't read those posts, I use the term "progressive escalation" to describe:
The tendency for submissives to grow more submissive over time and Dommes to grow more dominant over time.
I believe the primary reason for this trend is that the activities we regularly partake in become routine and we wish to "push the envelope" in order to obtain the same intensity of feelings as before. Dommes want stronger Domspace. Subs want deeper subspace.
In many D/s relationships the man has harbored submissive feelings for a much longer time than the Domme has held dominant feelings. At the start, this often causes some stumbling blocks to form as a man may have 10 years worth of fantasy and submission under his belt while the woman is forced to start from square one. The result is the appearance that men grow in submissiveness faster than women grow in dominance.
I think much of this is rooted in a Domme coming to an understanding about dominance within herself and submission within her sub. She must wade through, process, and then shed preconceived notions of dominance. She doesn't need to be a leather-clad whip-wielding dominatrix overnight and if that's not her thing, she doesn't need to be one at all. She has to see/read about intense D/s interactions that probably took years to build to as if they are normal and open to beginners (chastity, cuckolding, etc.). She has to face up to the challenges of finding things that appeal to her and then nurture them. This is a gradual process and not an on/off switch.
With her sub she has to get over the initial shock of his fantasy intensity... the "OMFG he wants me to do that to him?" There's a learning curve for understanding his psyche and subspace. There's fear that she will hurt him and do things that he won't like and will make him reject her. There's the responsibility of being handed the keys without a road map, yet she is expected to take them to the land of bliss and glee. These all lend themselves to making her feeling insecure.
During this early period, you can expect lots of topping from the bottom. Lots of guidance on the sub's part. She is finding her legs. She is learning to walk before she can fly. There's a good chance that much of this has her feeling tentative and unsure of herself, so much that even if she would enjoy this she probably isn't enjoying it now, nor being turned on by it.
Through this period the sub's desires seem to progress faster than the Domme's. I will refer to this as the "learning period."
Fast-forward a bit and make a few assumptions...
Assumption 1: The sub has not thwarted the Domme's progression by resisting the activities and actions that he doesn't like.
Assumption 2: She hasn't succumbed to the protocol generally taught in the "consensual BDSM" realm.
Assumption 3: The relationship has gravitated towards more of a "meta-consensual" D/s lifestyle. She has taken the reigns, embraced this role, and the D/s dynamic has gone beyond the bedroom doors.
These are some rather large assumptions. The first one carries quite a bit of weight. If he stands firm early on and refuses or resists taking part in activities that she wants to try under the guise of limits or that pleasure should be "mutual," not only will he halt his own progression to deeper levels of subspace, but he may derail her progression into a completely empowered Domme.
The second assumption may or may not happen, it depends upon how the Domme has chosen to educate herself on the lifestyle. If the first and second assumptions hold true, the third often follows.
Under these three assumptions (especially the third one), the relationship has now entered the "growth period." This is really where a Domme comes into her own and finds her unique identity in domination. She no longer hesitates to try new things, worries less about whether or not the sub is enjoying it, and knows what drives both her Domspace and arousal. This is often where concepts begin to drive her experimentation as she evaluates her own ideal of the D/s dynamic. e.g. She should have FULL control over his orgasms and denial and it doesn't get any fuller than a chastity device. If she wants to tie or chain him up as part of a punishment, it should be in an uncomfortable and unpleasant manner, etc.
In the growth period, the sub starts to fall behind. The Domme will start introducing things that "shake him up," and take him beyond his comfort zone. The sub may resist at first but eventually will comply quickly through strict discipline. She will likely take any resistance as disobedience and possibly even as an insult. He will be put through the wringer.
What really signifies the growth period is that the Domme will become more confident, more demanding, and more intense while the sub is forcibly ripped from his comfort zone, pushed into deeper states of subspace, and may become agitated at the unexpected nature of what will come next. Basically, her dominance grows faster than his submission.
Eventually... his submission will catch up to her dominance. The male creature is pretty good at adapting to almost anything once his ego has been broken. He comes to trust her judgement and knows she loves him and won't subject him to any permanent damage. When this happens they enter the "plateau period" of the relationship.
During the plateau period things go very smoothly unless the sub slips up. Another assumption: the sub has fully acclimated to the new lifestyle dynamic and things go smoothly. While it might seem harmonious, there is a fatal flaw for the plateau period: boredom. Things get routine, stale, and predictable. This may be the shared feelings of both parties or it may be one-sided. If it is one-sided, chances are the sub is fine with the arrangement but the Domme has started itching for more.
Both cases tend to lead to a similar outcome: things get shaken up and the relationship is brought back to the growth period. The only difference between how this is instigated is whether it's simply the Domme's wish or if it's the decision of both parties. In cases where an extreme decision is made, such as converting the relationship to a poly arrangement with additional subs, cuckolding, etc. it is often (but not always) consented to by both parties. I would be willing to wager that the more cycles between growth and plateau periods the couple has experienced, the more likely the Domme is to make a unilateral decision and simply tell the sub how things are going to be from now on.
The speed at which any changes occur varies greatly from couple to couple but barring situations with a commitment to the "consensual mutual pleasure" line of thought, it seems D/s relationships almost always progress each party involved undergoes their process of progressive escalation.