I guess there could be several explanations - mechanical failure of the drive gears perhaps, but I'd be wondering if the operating switch became defective and due to vibration in motion just powered the jack down as you were traveling?
I suspect it is a pretty rare thing and wouldn't be too concerned about it happening on your new unit, - but you could always just pull the fuse if it bothers you.
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.