Yet another possible cause is frame flex. In some frames the shifter cable is routed under the bottom bracket with part of the shifting cable exposed. In this kind of kike, the cable has not full length casing, instead the casing goes from the shifter to some cable stop in the frame, then the inner cable goes to another cable stop and enters another casing length finally going to the derailleur.
Some frames can flex under stress and pedaling hard from a full stop can be enough to momentarily deform the frame so the distance between certain cable stops changes, thus changing the cable position inside the casing which in turn changes derailleur position by just millimeters.
This position change is too small to complete a full gear shift, but is enough to make the chain "want to shift", hence the rider can feel skipping or "ghost shifting".
The solution for this is either create or use a different cable routing (Some frames may have alternate cable routing) or simply use a single casing from the shifter to the derailleur.