Two possiblities:
1) It was hit by something
2) It wasn't and the blades failed for some reason.
I favour the second, possibly metal fatigue or poor maintenance.
If something hit it then by definition it was a UFO as the flying object was unidentified. However I doubt very much that it was an alien spacecraft. I cannot believe that if aliens are here that they would behave in the manner popularly assigned to them. Skulking about, abducting only the most stupid and gullible people, doing anal probes(!) The amount of time and effort required even to get from the nearest star would preclude them popping over to act like irresponsible teenagers.
No, if aliens should ever arrive we all would know about it.