Getting sideways off jumps when you don’t intend to is 9 times out of 10 because you’re hitting an uneven face upon take off.  Think of a quad when it jumps off an uneven face; like where the two left wheels are higher than the two right wheels, your going to get sideways big time.  It’s the same kind of an affect with a M/C.  If there is a rut, a dished out area, a little rock or anything uneven about the place where the tires actually leave the ground it’s going to make you go sideways. 

The best way to solve this problem and keep your short clean is to make sure you take off in the center of the rut or dished out area, make sure you’re taking off on even ground.  If there is no even ground you have to compensate for the sideways pitch. 

The other possibility is that you're weighting one foot peg way more then the other. 

This is explained in visual detail on my "Basic Jumping and Launching Technique" DVD download at: www.mxraceschool.com

I hope this helps. 

Keeping an attitude of gratitude,

Gary Semics