Bottle bombs!!!! Hit the deck!!!
Welcome to our world. Bottle bombs can come about from unfermented malt sugars (incomplete fermentation) or from unbalanced priming sugar. The best way I know to ensure an adequate, balanced mix of priming sugar in your beer is to boil it in a cup or two of water and add it directly the bottom of your bottling bucket BEFORE you fill it up. If you're using a racking cane or auto-siphon to fill up your bottling bucket, just run the bottom of the tube straight down and let it rest on the inside base of the bucket. The flow will very gently stir the beer as it fills from your primary/secondary into the bucket. Avoid splashing or aerating your beer or oxidation can creep in.
There is one other cause, and that's a gusher bug. A gusher bug is a bacterial or wild yeast infection that can either be limited to one infected bottle or in an entire infected batch of beer. Basically, the bug will continue to consume normally unfermentable sugars and whatnot in your finished beer until the CO2 pressure causes the bottles to explode. They're usually caused by unsanitary bottling procedures, and don't think anyone is immune. I cracked open a 2012 Cigar City Brewing Improv last weekend that had a gusher bug, and it spewed out the entire bottle all over my kitchen in three seconds flat (faster than I could get it to the sink).