Police procedurals I "watch" write characters out of shows between seasons all the time. The remaining cast awkwardly bumbles through some explanation of weird off-screen events. I Google and the actors contract was up and they moved on to other projects, often for more money / bigger roles.
Googling Dr. Kroger's heart attack explaining his sudden absence from S7E1of Monk: actor Stanley Kamel died of a heart attack.
Some Google results are surprisingly sad.
@deafferret oh yes, that made me really sad, too, when I found out