its not common at all but these engines are just getting to be old enough to start seeing failures, in a 22R you would almost always see the overheating, chances are good the headgasket failed at just the right spot that it leaked the coolant in, another good chance is the as you killed the engine it was on the intake stroke and that little bit of vacuum left when you killed it was enough to suck coolant in
i doubt the head cracked but its possible
post pics when you get it pulled apart
