Bed bugs usually stay close to their hosts, but once there is no longer a food source that can nourish and support them, they will be attracted by food cues in adjacent rooms or units and spread through wall voids and ceiling holes, as well as unsealed bottom plates and utility pipes.
Another reason for leaving has to do with the way bed bugs mate. It’s called traumatic insemination. Males simply stab females in the side with their reproductive organ and inject their sperm, which makes its way to her eggs. Females recover from one mating, but several matings increase the chance of infection and death. Females will try to get away from groups of males to avoid being stabbed to death and in the process spread to adjacent rooms or units.