No one is sure. The only thing we know is that in the finale (and not before), the CG model of SOME of the Miranda class starships had the aft torpedo launchers lit. No dialogue, nor anything in the DS9 tech manual or any novels or comics (that I know of) mention it.
I see this as a goof. If anyone ever tracks down whoever the CG artist who animated those specific shots, we may see if they remember if they did that deliberately, or was just animating it because the aft torpedo launchers looked like impulse engines and didn't bother confirming it with an actual Trek nerd (I believe this is what happened). That person also gloweyed up the warp nacelles too, which is not consistent with most other shots of the ship, and I consider this also a goof. We're just left to find a suitable explanation that fits our own personal head-canons, and which can be presented to others as such since there is no hard evidence. And of course, this is the sort of thing we THRIVE on!