Is it possible your fermentations are stalling? You're reading, say, 1.017 and inferring that it is done, even if it has a few points to go. Two gravity points is all it takes to give 2+ volumes of CO2. Why might it be stalling? Are you fermenting at cool temperatures?
Maybe the yeast is floccing before finishing the fermentation.
I once had a Belgian Ardennes that "ended" at 1.016. I thought that was a little high, but figured, "What the heck, it's stable." So I bottled it and had exactly the scenario you are presenting. The bottling process and adding simple priming sugar must have roused the yeast enough to create volcanoes in the bottle.
Maybe you can measure the gravity of one of your bottled gushers. Just let it sit a while in the sample tube to off gas most of the CO2. Compare that to your measured finishing gravity to see if it has dropped a few points. If this is the case, you will need to take measures to ensure fermentation has really stopped. Swirling the fermenter and warming up the beer are two ways to accomplish this.
If you had an infection, you would probably be able to detect it with your taste buds.