[Wavytone]

There is one possibility.

These blades are known to fail eventually from stress fractures developing due to the massive forces at the root of the blade as it swings through each revolution. If one blade failed its quite likely that in doing so it collected the next blade, the piece that snapped would be both heavy and strong enough to cause the massive bend seen in the photo.

It takes an phenomenal wallop to bend one of those blades, let alone break one - no birds even come close to being big enough, and there is the question of debris on the ground - more the lack of it, apart from the broken blade.

Gotta love the way people immediately leap to assuming UFO's or aliens did something, rather than look for the rather mundane reason. Von Daniken would be very pleased.