We live just across the channel from Koh Samui on the
beach at Koh Pha-Ngan, about 18Km from Samui airport with a clear view of aircraft approaching from north and north west directions.
Just before 2pm, a very dark-looking front begun rolling in from the West. A few minutes later it hit us with gusts of approx 30-40 knots. Rain was falling horizontally, and coconut tree branches were being thrown about like feathers.
Looking at the ThaiVisa forum pic, I can see that the aircraft approached the runway from the southern end. With strong westerly gusty crosswinds forcing the pilot to land at a steep angle to the runway's orientation, it's my opinion (as a non-pilot) that as the plane touched down it could not straighten fast enough and consequently veered off across the grass strip into the old fire station near the control tower.