What would cause a hardwired smoke alarm to give one short beep when appliances are turned on?
The detector beeps because the momentary drop in voltage (or aberration in waveform) at the detector is sufficient for the detector to pick up, perhaps interpreting drop/aberration as a sign that it is about to lose power.  

Might be a:


hardware/software bug in the device
feature for letting the owner know that the detector is functioning properly 
feature that warns of a low battery (the battery that powers the detector during black outs, etc ...)  
feature to indicate that the detector's service life or warranty has ended
sign that the circuit is excessively overloaded
sign that the detectors are wired improperly