broken spark plug resulting in compression loss, cause?

[gpeele]

this is on a 95 rps13 blacktop motor that was hesitating at part throttle. i found the cause was a spark plug that is securely in place but the top porcelian part that attaches to the coilpack boot is cracked and wobbles around in the metal carrier resulting in a compression leak, also the whole spark plug well has a thin coat of black soot along with the coilpack boot. i havent been able to take out the plug yet because i need to borrow some tools. ive never heard the engine knock since i installed it and the pfc was reading peek knock values of around 15-20 before the engine hesitation started.

i guess i want to know if anyone has experienced this and what could have cause the porcelian to break like that.