Heres how I understand it the bacteria that do the final stage of converting nitrate to free nitrogen gas are not 0 oxygen bacteria but low oxygen level bacteria, if the conditions of the bed turn to 0 oxygen you get a different bacteria develop that produce sulphur dioxide and this takes out the whole tank! The critters keep things turned over enough to stop completely anoxic conditions developing. Theres also an in between stage problem as well where nitrates are converted back to nitrites when things get disrupted.