its neigh on impossible to bugger the thread on a main bolt its fucking huge! and you are torquing into cast iron! (on which there wont be much strain anway)
i cont really see any reason for it 'falling off' unless it wasnt correctly torqued.
What bolts were they? standard or A.R.P?
If A.R.P then they prob need more torquing just like the big ends do!