Broken porcelain on spark plug??

So the Mrs. and I were out doing some "pre-running" of the route to meet up with Motorhead for the flag hand off next weekend, on both the old GPZs
And upon our return, I found that a couple of the porcelain insulators on the spark plugs of the big bike were broken? WTF :blink: The bike seemed to run fine, and replacements were quickly installed, but I have never seen this happen before. Any ideas?

Had a plug break on the wife's Saturn one time. Have no idea why it happens. I assume on a bike it's from hot/cold cycling along with vibration.