I read one theory to why these attacks happen that seems plausible to me. The Mediterranean has a lot of noise from boats that disturb the marine life. Imagine living 24/7 in a construction site. I think they are angry and desperate.
I'm really not sure that's the reason. Someone else in this thread had an idea that they were trying to replicate a prior result (obtaining something desirable from a disabled yacht) but I'm inclined to believe they're just being dicks. And why not? We hunt for sport. And we know that orcas engage in play, even quite cruel play with cornered seals. It could be adolescent orcas challenging each other, like pelagic ding dong ditch.
EDIT: The boat in the article was motoring, not under sail. So strike the word "silent" for "a lot quieter than a motorboat."
This has a map showing attacks and they are definitely also happening in the Mediterranean.