H'mmm, I'm sure that there will be someone along soon who's better qualified than me to comment on mountain bikes , as I'm an old "roadie", but one common problem is adusting the axle so its too close to the end of the dropout. This means that the clamping surface of the skewer is not fully suported, so when you apply torque, it is able to pull past the end of the dropout, so you should always have some of the dropout visible past the skewer (blimey, that didn't make much sense even to me
)
If you are a bit of a power house, the answer is to get a frame with vertical dropouts...this means there is no wheel-base adjustment, but you cannot force the spindle to move, as the only way it can go is vertically down.
At the end of the day, Q/R back wheels pulling out of alignment is a fact of life if for the really strong bloke, that's one reason why you don't get Q/Rs on Track bikes...the force that some of those "incredible hulk"sprinters apply is pretty bl00dy scary.
Anyway, if its not the "spindle too close to the end of the dropouts" problem, you'd better wait till someone who knows ehat they're talking about comes along....is it Gizmo that's the mountain bike guru?