flap is retracted once get on the ground
That is fine, I am using this technique as well. If anything, it will put more weight on the wheels, improve stability and improve breaking action.
I suspect your pilot got into this accident because he touched the tiller (nose wheel steering).
The bottom line is this: do NOT use nose wheel steering -- do NOT touch the tiller -- during the takeoff roll and after landing! Only use the rudder pedals!
PS. Separately, I heard of a pilot in Gabon who landed long and too fast, bounced several times, then lost control. The aircraft ended up in ditch, upside down, only minor injuries.
The Twin Otter is the best aircraft I have flown, it has amazing performances and capabilities, it is easy to fly, but it will bite if not handled properly, or with inexperienced pilots.