It could be a minor infection; something that ate just a little sugar, added carbonation, and not a noticeable flavor yet. Possibly a brett? Give all of your plastic a good soaking in PBW, then Star San. If your bottling bucket has a spigot, do you take it apart and clean it every time? I don't think there's much that can survive Star San at a pH of 3.
Also, could it be possible that the beer wasn't at terminal gravity yet? I've had a beer stall and look done, only to ferment a bit more in the bottle, resulting in over carbonation.