Hey LennyW, it all makes sense now. The bike thought he had right of way because of him going straight ahead. Little do Thai folks realise that the very small funny shaped red sign with English word of STOP means anything ( due to the solid white line across his side).
I see same situation many times in Thailand. Driver thinking he had right of way and legally had, was pissed that he was honked for no reason and then once hormones, rage ,whatever took over, the end result was a certainty.