If I had to guess, no reverse gear mounts to support the weight of the gear, then you add the fact that there are holes in the bell housing for air cooling and you got a weak link and a point of failure.
That is good stuff. Aluminum is pretty soft stuff even cast which the bellhousing looks to be. Odd that it would break in such a brittle fashion. Looks more like a broken cracker than bent metal. Makes me think some vibration fatigued the shit out of the metal for a long time and then it just gave under a particularly high load.
I see rear motor mounts holding up the back of the motor. I see the tranny sitting on the bottom of the boat. I don't see any tranny mounts that could have prevented this overhung load on the aluminum bellhousing. It must have been fatigue failure over a period of time, or maybe all at once during a grudge race?! The shaft had to move quite a bit when this happened. I can't see the shaft coupling or stuffing box. So what kept the boat from sinking? Any pics of her on the bottom? :-o