I think there's a lot of factors at play with these types of situations. First off, head coaches and coordinators have an interesting dynamic. Obviously Purdue's offense progressed throughout the years and was not what it was in 1997 by even 1999. Maybe they were in solid agreement on the transformation of the offense, maybe they weren't. I believe they were.
Secondly, some people just need a change of pace. Since leaving Purdue in 2005, this will be Chaney's 6th different team. When you're with the same people, the same system, the same program - it can be hard to freshen things up.
Third, I'd say that people often look for scapegoats in the wrong places. It happened this year - the blame gets directed towards coordinators, not the head coach. Do I think Spack/Chaney got put in these situations? Absolutely. Does that mean they were doing a great job though? I don't know about that. But I think Tiller had obviously built up a lot of love that it was easier to go after those 2 when things weren't as rosy. Obviously looking back, Tiller kind of lost his fire and it showed not only in terms of coaching, but the recruiting just wasn't as sharp as it was.