The headset was well and truly wedged in this one, even, with the expansion from the crack, so i think an oversize headset being wedged in might be the answer fro this one- I measured the headset and it was just over 1 1/8
Now we've established they have a tendency to crack in the headtube the question we must be asking is why?
Suspension fork ? Rough riding ? Must be some common denominator
This isn't a problem reserved for Pace frames though, plenty of other manufacturers produced aluminium frames which also seemed prone to cracking at the head tube, amongst other places.
Maybe the type, grade, and thickness of the aluminium used is the issue? The fact that the RC300 came with a butted head tube also says quite a lot about the construction of the head tube itself.
And lets not forget the finite fatigue life of aluminium alloys as opposed to steel or Titanium.
I do think there is an element of user error or abuse in some cases though. The Pace frames were designed for XC/ race use, so getting "insane" air or hitting "gnarly" drop-offs is perhaps exceeding the design parameters of the frame, pretty much any frame will break if you drop in from a sufficient height.
And the increasing travel of suspension forks no doubt contributes to both the abuse the frame takes, and the stresses put on the headtube on a daily basis.