The latest report I read in Scuttlebutt says their engine was running, too. This further suggests to me they were maneuvering to join up with another boat. (I would assume they had a generator to run the computers, so why would they be running their engine?)

The only other scenario I can think of besides kidnaping is that they got off course and ran aground on a reef and started their engine to get off the reef and couldn't, so they all got in the water to push the boat off the reef and succeeded but then couldn't get back on the boat because it took off without them, with sails up and engine running. Is that possible in that area?

Were these guys all currently married, or were they single? That information must have been in the news reports in Australia.