Hydraulic tensioner base plate broke in half!

I replaced 2004 330xi's mechanical tensioner in late 2012, and it's been doing fine until a couple days ago. I started the car after it had been sitting for a week, and there was a loud squealing. I slowly rolled down the street until I heard a thunk, lost power steering, and the battery light came on. A quick search of this site told me it was the belt. I was able to pull over and stop safely. When I opened the hood, I found both belts wrapped around the fan, and the tensioner pulley at a wrong angle.



Once I got the fan out and examined what happened, I found the hydraulic tensioner's metal base plate (#2 in the diagram) broken in half, and the top mounting bolt broken just above the threads. (#11 in the diagram).





What happened??? Are belt forces strong enough to pull this apart? My big concern is that there is something wrong which caused this, and may cause it again.