Whatever.
I know nothing of the specifics of the situation but if any of the comments on here are accurate then it is reasonable to assume some of the following may have prompted this change;
A) As a new coach Stitt was obligated to retain Ty for some continuity. He may want his own guy.
B) If there was tension (likely) then each may have been wanting a change anyway.
C) Ty's wife just got a job in Bozeman. This could explain his jumping over to little brother.
D) Coach Choate needs huge instant help. Finding a decent D coach with Big Sky experience is huge.
E) Despite the talk, Choate doesn't have MT connections (beyond playing at Western). Ty does.
F) Cat's defense was worst in the nation. IF TY DOES ANYTHING HE WILL LOOK GOLDEN.
G) Ty probably got a pay raise and a great resume builder showing he can turn a defense around.
Gregorak did a good job in Missoula and always had a decent defense. On the other hand he also had some exceptional talent. With this he consistently fielded a good defense but not a great one. I hope he does well in Bozeman. I expect he will make their D respectable again. This will help him if he wants to climb the coaching ladder.
Ty is a good coach and I'm sorry to see him go, especially to Bozeman. Yet as I began this post, "Whatever".
I think what I'll miss most is the guy's passion on the sidelines. I'm confident we will find another defensive coordinator who can take the talent on the Grizzlies sideline and field a solid defense.