Its really not completely understood. Its not always preventable, and most times it probably is genetic especially if it occurs at an early age and you know there has been no serious injury, but injury can cause it.
http://www.vetsurgerycentral.com/legg_calve.htm
RIght now I have my lil guy on pet savers, but Im not real sure I like the effects, but then again, Im looking for perfection because I dont want him to go thru the surgery but that is probably what will happen