Sometimes, you don't even need a manager-change for your job to be in danger. I know a guy who had a manager who was pretty cool and a good mentor to him. And then one day, something happened to the manager and he stopped mentoring him. No, he stopped interacting with him. Frequent lunching together came to a screeching halt. Eventually he was left out loops, projects and eventually out of job.

He still doesn't know if the cause was one of these:

1. manager broke up with girlfriend (which did happen) and started not caring

2. my friend said something about politics that offended him?

3. the manager saw 1 mistake by my friend, and just decided he didn't want to waste his time on him.