I have one coaching spot to fill, AAA hitting coach.  There is exactly one guy available and his hitting rating is 49.  The best AA guy to promote has a hitting rating of 59, and there are 15+ ML level coaches with ratings in the 70's and 80's who obviously will not accept my AAA offer (stupid part of this game).  If I don't offer the job to anybody will I get the 49 IQ guy who is left or will the AI convince one of those ML guys to take a AAA job?

A ML coach won't drop unless all the ML jobs are filled

Ive seen coaches drop down to AAA... if all ML slots are filled or something.  But I can't pin point it.

If it happens, it will happen on the very last cycle.  It does not happen every time though.

I think I will wait and see what the AI does for me.  I just looked at my AAA roster and I don't plan on having one single meaningful prospect hitter at the AAA level.  Loaded with solid pitching prospects but not one future ML hitter at the AAA level.  Therefore, I will let the AI make the hire and if I end up with a 49 rating dude it won't hurt my franchise one iota.  

Wait until the final cycles.  If there is a ML Hitting coach or 1b/3b coach that didn't have a job last year, they will often accept if you can offer more than the minimum.