My fist suspicion was Nagara, too, but then I blew up a medium modern split merchie with one contact hit the other night.
Also, is it at all possible for a contact shot to set off the keel? I know you said it wasn't before, just double-checking with you.
Another possiblity is that those of us using RSRD are seeing alot of ammo-laden ships in our patrols, accounting for what seems to be extreme volatility of marus in general. IIRC, in the stock campaign layer no ships have cargo.