Typically a beer over carbonates because it is stored in too warm of a location for too long. Yeast will continue to ferment if left to its own devices. Once a beer is carbonated (typically 2 -3 weeks) make sure to move the beer to a location below 55 °F for storage. At this temperature the yeast will hibernating and stop creating CO2.
Other possible causes are either too much priming sugar was added and/or the beer was bottled before fermentation was fully completed. In order to ensure a homogeneous mixture of your beer and your priming solution one can use a sanitized stainless steel stirring spoon to gently (careful not to add oxygen) mix the beer and priming solution together. Alternatively, pour your priming sugar solution into the bottom of your bottling bucket first, then rack the beer on top of it and this will mix the solution in evenly with out the risk of oxidizing it.