Sounds to me like something came off in flight, like an aeiloron or something like that. Could have even been a motor mount, which would cause it to either go left or right, depending on which side it was. If it wasn't those two then partial wing came off, but there would be barely any lift.
I also don't think that you kept it in the air for 15 minutes, because I know some of the most powerful batteries can only last about 15 minutes, and you also said that you were flying it before, so I think it's doubtful that you did keep it in the air for that long.
Put this in the O'Club
<S>