Interviewing for an academic position after abusive department chair in previous academic position
You can honestly say that the former position was "a bad fit" personally and professionally and that you seek a "more compatible" environment. I've been in enough strange situations that I suspect people are aware that they exist. There are enough similar questions here to indicate that academia isn't utopia. 

You needn't speak ill of the former head. But if the "environment has become less conducive to personal and professional growth" it is likely a fair assessment. 

Also, just having been promoted is an excellent time to "look for other, more challenging, opportunities."