Why did my bottle explode?

I made a clone of Left Hand Brewing Milk Stout using a recipe in Clone Beers. It came out great! But about one in 10 bottles seem to be really over carbonated and yesterday one exploded. They were all bottle carbonated by putting 1 1/4 cups light dme (boiled in 2 cups water) in the bottling bucket. The rest of the bottles came out with just the right level of carbonation. Anyone have any speculation about how some of them got to be over carbonated?