^Agree, but another huge factor IMO is the supply of new artists/material - in 1996-1997, 80s artists like Phil Collins, Sting, and Rod Stewart had pretty much exhausted their sound, while there was a huge increase in pop/alternative artists like Sarah McLachlan, Sheryl Crow, Dave Matthews Band, Hootie & The Blowfish, Alanis Morissette, Jewel, etc.
If Hot AC had stayed "soft", by 1997 they would've had very little new music to play - and the 25-34 audience at that time seemed much more interested in Alanis and Dave Matthews than in Bryan Adams and Rod Stewart
Formats shift IMO because of the supply of artists - same reason Alternative shifted two years ago, in that most new material was now being put out by Indie/Folk artists, and the supply of harder-edged Active Rock bands was dwindling