If you were actually tacking (ie going thru head-to-wind) then it's really odd that there was enough shock loading on the boom to do this damage (barring a weakening of the boom).
However if you were gybing from beam to beam then in fact the loading could be significant if not controlled.
I wonder if in fact the loose footed sail is a large factor - I'd like to know if anyone ever actually analyzed the boom loading of loose footed vs bolt rope mains.... The major loads will be on the clew and tack in any case, I suspect. Esp with the outhaul eased I wouldn't expect the bolt rope to really spread and absorb a lot of load. But who knows......
Also odd that 2 of your blocks were installed in the provided slot, while the other was through drilled... doesn't sound like a factory/rigger install.