Sometimes through the late commander and early captain ranks, some of the enemy ships were apparantly fitted with corbamite devices.
This is most noticable in the 3 frigate groups, and Hirogen are the ones I've noticed most commonly.
An example setup, I pound one of them apart and it blows up in short order. it's a -2 after all. When it pops, man does it pop. I'm 4km away, and the explosion takes me down to a few % hull through a mostly there shield (science vessel). It wasn't entirely detrimental, since it blew up BOTH other frigates (one which was on the other side of me from it).
This is random and sporatic but it's frigates and the occasional science vessel I notice it on. (I did have one romulan battleship that blew me apart while I was braced for impact but I was right on top of it with a down shield and 20-30% hull damage at the time so that may be more normal)