Nuisance smoke alarms
Some battery-operated alarms use a radio signal to interconnect so they're all set off together. You should be able to verify/test this using the test button: press it on one, and if the others go off then you know they're interlinked.

As far as what's triggering them, there's a few possibilities:


Dust in in the sensors. Recent renovations especially can cause dust to get on the sensors, making them very sensitive to even a slight change in the air. Using a vacuum to clean them out can often remedy this.
Signal from other nearby units. If you have wireless interconnect, they may be on the same frequency as neighbour's alarms. There should be a method to change the code they use (often some dip switches on the back).
Bad location. Smoke alarms should be located a minimum of 4" away from a edge of the wall/ceiling, inside corners, and the peak of a vaulted ceiling to avoid the dead air space. Good airflow is important. 
Wireless interference. Apparently some brands can be sensitive to interference from other electronics, and it's best to relocate the alarm or electronics. 
Defective alarm. It's possible one or more alarm is simply defective. Most smoke alarms have a 10-year age limit.