By NAPAT KONGSAWAD (Associated Press) BANGKOK — Aviation investigators arrived in Bangkok Wednesday to learn how and why severe turbulence sent a Singapore Airlines plane into a sudden dive that tossed passengers and crew around the cabin, leaving a British man dead and dozens others injured. Twenty people remained in intensive care in hospital after Flight SQ321, which was flying from London’s Heathrow airport to Singapore, hit the turbulence Tuesday over the Andaman Sea.