Definitely not a bug. My assumption is that it's Explosive from an enemy ship. You're supposed to get knocked down when you're hit by it too (only works in SvS for some reason...).
Explosive might be supposed to have an X percent chance of damaging the opponent's cannoneers. Obviously this only works when it's thrown at you by an NPC ship (except in SvS). If I'm correct, there really should be a message telling you that in-game...