Hey now that's a weird one. It appears to be purely a kidnapping for ransom. The pirates didn't even want the boat itself, just took the people.
Although I am on record as saying that the possibility of being a victim of piracy is vastly over stated I cannot undertsand why anyone would choose to take that route through the Red Sea in this day and age.
Admittedly I don't know the statistics on how many yachts go that way each year v how many get attacked.
BTW, In this particular incident they had run low on fuel and were approaching the Somali coast.
“Give a man a fire and he's warm for a day, but set fire to him and he's warm for the rest of his life.” Terry Pratchett