flex disc bolts sheared off at transmission

[biopete]

This is a 1993 300D in my shop. The owner had it towed in because it had a loud rattling and wouldn't go anymore. I got it on the lift this is what i found. WTF? I just replaced that flex disc and balancer about 4 months ago. What did i do wrong? You can see the direction I put the allen bolts that came with the flex disc. Should they go the other way maybe? I was careful to get the flex disc guide pins into the transmission propellor and align the balancer with the mark What would cause this to happen? Lucky, no damage to anything else was done.