I'm definitely guilty of keeping my grains and beans too long; on more than one occasion I've bought red lentils, only to find I've already got a whole bunch in the cupboard.
But that doesn't explain why things that lie dormant for months come to life while we're away! Is it the lack of air circulation, as doors and windows stay closed? Does the house get hot, providing the right incubation conditions? Does anyone have any ideas?