Blown Thiel 3.6 tweeters/midrange

I'm hoping some you Thiel owners can help me out on this one. A friend of mine blew the mid-range and tweeter on BOTH his 3.6's over the weekend. He went to the kitchen to fix a drink and when he came back there was no music, just a muffled thump from both speakers. Apparently the voice coils have actually pierced the diaphragm and you can hear faint music only if you put your ear close to the drivers. FYI the 3.6s were being driven by an Aragon 8008 ST / 28K fed by a Pioneer PD 65. Any feedback on what could have possibly caused this would be MUCH appreciated!