I would guess (I'm not there) that someone went through the open line on the way to the next mark, someone on the RC hadn't been counting laps and thought the boat was finishing, so they blew the whistle. Boats in earshot hear the whistle and figure they misread the course, and bore away hard (if beating) to go through the line, too. Boats not in earshot (or more confident) continued on. Boats that altered course probably dropped some positions, assuming they came to the finish line while the RC was figuring out they had screwed up and told racers to continue sailing. If there wasn't a BIMBO taking roundings, it might have been tough to reconstruct the race in a fair way so that only the boats requesting redress got satisfaction.

Bottom line - there was an RC action that caused confusion. It very well may have been the fairest thing to toss the race. Armchairing it from out here on the internets may be engaging, but it is impossible, IMO, to second guess this one with any sort of confidence.