[bandito2]

I would think it broke because of a flaw as well. A small flaw in cast aluminum allowing a small crack is kind of like a small tear in paper. Under under repeated cycles of compression and tension the crack will open up very easily. Much like how a sheet of paper is so much easier to tear once a very small rip has been started.

Nicks on an edge, irregular or acute (sharp) angle edges like a square opening present areas for stress cracks to appear and propagate. Cleaning up burrs and mold flashing, rounding the sharp edges of cuts and drill holes, smoothing surfaces and making bends with some radiusing rather than sharp angle bends are ways to help cracks from starting in the first place or to prevent a crack from getting larger.

Like Dodgeme said, some flaws are too small see initially or may be in a hidden area. Like inside of a drive-line case for example. The broken case you experienced may not even be your fault.