CBS was top-heavy. When Entercom took over, they began cutting the "weight" almost immediately, perhaps without taking into account the expensive people they showed the door were responsible for CBS being a "cash cow" in the first place.
The other part of it (and this is common with mergers) is that the CBS people got paid more and had a better benefits package. When they merged in with Entercom, there was resentment to that, especially in markets where former competitors were being merged together in the same building.
There was also some integration with the TV stations that was difficult and expensive to break.