Telling you how it might have happened won't help you with your current problem of getting it fixed, but these are probably the two most common sources:
1) The powersteering pulley contacts the subframe. Occurs when the car hits a large pothole or other impact, or when the subframe bushings are worn out and a smaller impact. The power steering pulley gets damaged missing the edge and then the belt comes off. Check your powersteering pulley for damage.
2) The OFHG leaks oil onto the tensioner pulley. This also weakens the belt as it absorbs the oil. Eventually there is not enough tension to keep the belt running true and it slips off. Look for leaking oil on/near the tensioner, on the belt itself, etc...
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