After exiting the trailer he likely tried to put in forward, but instead left in neutral. I do it all the time, but fortunately for me I don't have docks to worry about. The worst I ever screwed up was running the back wheels off the concrete and into the grass. Just used another forklift to put a couple palettes of shingles on the other lift which weighed it down enough for the drive wheels to make contact and drove it out.
My piggyback lift on the other hand... well, that's another matter entirely. :lol:
