We know that kromer did not outright quit, the parting of ways was not contentious, AND his son is still employed by the rams. Putting this together suggests that kromer did not have a huge issue with mcvay. Further the timing indicates that mcvay was not displeased with kromer's performance.
Yet kromer is no longer employed by the rams. This suggests that Mcvay and Kromer but heads on some issue. The timing to me suggest that it was personnel- or coaching-related issue and that mcvay wanted to go in a direction (rightly or wrongly) that kromer objected to. Thus rather than being in a "bad marriage" they decided to part ways amicably.
