Originally Posted by gometz
Unless you "kill" the yeast or remove it somehow, there will be a little fermentation going on until there is no more sugar for it to eat.
This is not true. Different yeast strains are able to consume sugars of varying degrees of complexity. For example, English and Scottish strains generally are less attenuative, thus will not ferment a beer to as low a final gravity as say, a typical American strain (or, in an extreme beer yeast example, Saison strains). There are MANY commercial beers that are bottle conditioned and unpasteurized that will not have this happen.
Once yeast has finished consuming the sugars that are available to it, it will be unable to consume more. Residual sugars will NOT ferment unless another microorganism is there to consume them. Brettanomyces, Lactobacillus, Pediococcus, etc, *WILL* consume these long-chain sugars but will take time to do so.
In short...if you've got "some" bottles gushing after the beer has been bottled for a few months or more, you've got an infection going on in those bottles.
If you've got gushers after a week or two, odds are the co2 produced during bottle conditioning hasn't been fully dissolved into the beer yet. Let the bottles hang out in the fridge for a few days and it will happen.