Also there are alot of other reasons of such behaviour.
-malfunctioning of sensors - when turning the wheels - ECU doesn't increase the revs enough to keep the engine stable due to wrong signals by faulty sensor.
-faulty power pump or alternator required more power of engine to operate - indirectly they are putting load on each other by decreasing engine revs.
Soemtimes u feel a little bit drop of RPM when switching ON head lights, AC and using power windows etc when car is on idle. Actually - u r putting load on alternator and it requires more power of engine result in drop of engine revs - and when sensor inform ECU - it increases the revs required to stable the engine.