Beer is created through the action of yeast upon the sugars contained within the malt. This yeast while alive and in an environment where it can survive will process its food. If you have ever had a beer where the yeast has not been filtered out may have experienced the results: gas.
Hazed and Infused, an unfiltered dry-hopped IPA by Boulder Beer Co., which is tasty most, is one such beer. Thank you BFS. You enabled me to sicken my wife on our morning commute after merely having one beer the night before.