2000 Volkswagen Jetta Hole Torn in Housing

Hi, I was driving down the road, when the transmission started jerking to the point where it was no longer driveable and I had to call a tow truck.
Now, I have a new transmission, and am to the point where I have the old one off, and am ready to put the new one in, but I thought that I had better be sure that it was just the transmission, and not something else that'll just do the same thing to this new transmission.
Probably half the housing is gone from around where the cv joints connect. A lot of the aluminum was ground up by the gears inside. The gears themselves don't look chipped or bad or anything, and everything spins seemingly without problems. Maybe something is loose inside and it wobbled enough to hit the housing or break something off then throw it out the housing? I don't understand what could cause a chunk of the housing to break off (and no, I didn't hit something. I had a small hole in the housing before this and assumed something just bounced and hit the transmission, and I just patched the hole, but now a huge chunk is gone in the same spot. So it couldn't just be that I hit something).
Have ya'll ever heard of this? Any idea what could cause it?