This decrease in active D/s is most often caused by a degrading power structure and lack of desire to follow and enforce protocols. Much like the four pillars a D/s relationship requires constant reinforcing of the structure in place in order to remain healthy and functional.
In many cases when mundane details of life become problematic a d-type will not wish to be bothered with reinforcing power structure, or an s-type will not wish to follow a power structure set into place.
A lack of reinforced structure will degrade a relationship over time. In some cases both partners may decide this is a good thing and move away from certain structures that no longer serve them and that is simply an evolution. In other cases the degrading structure will lead to lack of relationship health for one or both partners, causing undesirable conflict and challenge.
If both partners are happy with the relationship evolution there is no reason to resist the change. If both partners are unhappy with the lack of structure, in most cases this means both must put in more effort to reinforce the current power structures or become increasingly dissatisfied, potentially ending the relationship. In the case one partner is unsatisfied while the new partner is happy with the evolution it may be time to revisit negotiations.