Two things come to mind, though both seem unlikely. First of all, did you check the setting of the fader on your radio? I know this sounds obvious, but you didn't mention it, so I figured I would throw it out there. Yeah, it shouldn't just suddenly change setting from center to rear, but something weird could've happened.
The next possibility -- and again, I realize this is not likely -- is that one speaker was already blown (maybe the passenger side front speaker), but you didn't really notice it because you were still getting sound from the other 3 speakers. But then, the other front speaker blows, and suddenly, you notice that there is no sound at all from the front, which makes it seem like both speakers went at once.
Again, I realize that these are long shots, but they may be worth a look -- especially the fader, since that's easy enough to check. Checking for blown speakers would be more difficult because it would involve taking things apart.
