How many times has a coordinator with no HC experience been hired @ a P5 school in last 20 years??
Kirby Smart, Jeremy Pruitt, Mark Stoops, Mike Stoops, Dan Mullen, Shane Beamer (technically was never a coordinator), Clark Lea, Mike Leach, Pat Narduzzi, Manny Diaz, Jeff Hafley, Jonathan Smith, Justin Wilcox, Dave Aranda, James Franklin, Mel Tucker, Greg Schiano, and that's just a once-over not counting internally promoted assistants.
It's very common. The common denominator of course being big time success at big time schools as a coordinator.