ib nora, I can't figure that out either. If you go to the website of Bob Riggle, you can see that he made the transit with the convoy a couple of years ago. Actually, they don't write about crossing the Indian Ocean but they do write about the Gulf of Aden and the Red Sea - and the convoy, warships assisting, etc.
http://www.gaiaworldtour.net/Gaia%20Diaries/index.htm (about 1/4 of the way down the page, Gulf of Aden)
I don't think we'll ever know, but I'm sure it was for a good reason. I do know that someone had written that the route taken that goes the farthest from Somalia is 40% longer than a straight line across the Indian Ocean, meaning that you're out there 40% longer as a sitting duck. Perhaps they decided to take the quick route?
I'm leaning towards this myself, in the absence of any other info: Perhaps they had electrical issues and couldn't radio the others to tell them they had been separated during the night? They were not using the built-in VHF as it is stronger and carries farther - they were using handheld VHF radios only, which carry a short distance. And I believe they were using their radios as little as possible so as not to alert pirates to their location, so if they became separated, they may have thought they could find the others again without using the radio (which would put not only themselves but the convoy in further danger). The handheld VHF probably was out of range, yet using the built-in radio with the boat antenna would have carried far enough for pirates to pick up the broadcast. And an SSB or HAM would have been out of the question.
There is another pirate story that is kind of nice to read in the face of all of this, considering the folks got away and the pirates perished:
The 2nd entry... "8 March, 2005, YEMEN - Violent Pirate Attack On Two Yachts"
http://www.noonsite.com/Members/sue/R2009-01-21-5