Thank you, ItsAqua, for a prompt reply. For fall, he hit the mole mound on a small speed. I guess the front wheel just stopped hitting small obstale, and he fell forward (small speed, so it was no damage). I just wonder if with 2WD, the front wheel just pulled itself on the top of the mound.
Also, another obstacle we regularly have to overcome is lifting scooter up onto sidecurb stone. Currently, we have to stop, and manually pull front wheel up, then add throttle so rear is pushing scooter on the top. I was wondering if 2WD scooter can pull itself onto it.