In my opinion, cases where no maintenence has been performed, the valve adjustment could possibly be the culprit of the carbs blowing off....after all, if the valves can't open and close when they are supposed too, their could easily be a "burp" that disengages the carbs. If the valve clearance is only slightly out of spec, this should'nt happen though....only if clearance is way off. Actually, this should'nt be a reason for the carb "blowoff" problem since this is normal maintenance for a thumper and should be performed regularly. In my opinion, 99% of the time, if your have this problem, your idle circuit is lean.
My carbs blew off one time and the Raptor hung wide open in 2nd gear.....that was a wild ride.