When ever I have run across this it is because of excessive wear, usually on the bottom of the shifter. When it wears thinner it lets the first shift rail drop back into gear while going into the next gear. I have welded them back up and then ground back down to size.
If the detent balls are loose or missing it could cause a problem too.
My worst experience was with a VW offroad dune buggie. I was in the woods mowing down 1-2" trees making a trail. I had to stop and went to back up and the shift rail dropped back into first gear while the shifter went into neutral. I had to get a chain saw and cut it out and drive it back in first gear. It was an easy fix, I took out the trans fill plug and popped it back into neutral with a screwdriver.
