My 2 cents:
Threads in block not clean or liquid (i.e. coolant, oil) in threads or how about head not properly centered on block pulling bolts at angle. With no locating pins the bolts will try to center the head as tightened.
Related note, I did the reinsert the broken bolt to remove the remander on an 5.7 diesel I had for many years. Boy, those were the days!
