Also have to understand that when priming you only add some simple sugar to the brew to get your yeast active again - thus fermenting - but this time with the cap on the bottle so the CO2 produced by the yeast won't be able to escape - thus dissolve in the beer, godisood -!! I'm sure you're familiar with that, so why mention it? Because your yeast need be happy!
Do you condition your brew at the same temperature you had been fermenting it in the first place? If not, you have to! Only after carbonation is complete can you chill you're brew in the fridge.
Second, over carbonation means only one thing: yeast have to much to eat (unless using actual gas as a fizzing agent). Simple as that! Possible explanations from most obvious to others:
a) make sure your priming additions are what you need and not more;
b) wort not being completely fermented (yeast actually have a full-on buffet!), you have to monitor your brew's gravity and attenuation to have a reliable idea of the process;
c) possible wild yeast and/or bacteria infection (while bucket priming?) that would push fermentation beyond the expected;
d) possible over exposure to oxygen while priming which would bring the yeast back to an aerobic cycle (not sure about that one. as of now I don't know if the yeast produce different amounts of CO2 from aerobic than anaerobic cycles but I'm sure you can find that info if you search a little further);
e) too long or too hot conditioning? I've had a few bottles that a friend brought back to me which he kept at hot temp (25 to 35 celcius) for an extended period of time. They were practically all gushers. Perhaps the yeast had been going through a dormant phase for some time and then got active again? If so, what was it eating???
Good luck! "relax, don't worry, have a homebrew"!
Oh! A tip:
If you chill your bottles for about a week, take them out as you drink them, partially crack the cap open, let the CO2 come out of solution, crack it a little ore and so on until the bottle is open, pour it slowly into a chilled glass and let it stand until it reaches the desired serving temp, you should be able to enjoy it!