If it keeps happening, you can try some hi-temp Loc-Tite. It's like crazy glue for threads. Be careful not to get carried away trying to keep it tight and tighten the plug so tight you strip out the threads in the head...then you have a real issue! Or another old trick, if it happens again (this one doesn't cost anything), take your plug, take a punch or chisel and hammer, and mess up the last couple of threads a little, the ones closest to the top of the plug. That will provide some additional locking friction when you tighten it against the threads in the head. Only do the top one ot two so you don't risk messing up any threads down in the head. This trick, though I admit a 'Grapes of Wrath' kind of ploy, will work on almost anything that keeps vibratng lose. And on these engines, there is plenty of vibration!