The force acts across the area of pipe, usually a collar, embedded in the filter cake of a permeable zone. It shows up when the pipe has been still, the overbalance is high, and the zone is permeable. Jarring, a spotting fluid that shrinks the cake, or reducing the overbalance are the usual responses.
It is not mechanical sticking. Mechanical sticking is a tight hole, a cuttings bed, or a keyseat, and it does not require a pressure difference. Adding mud weight to a differentially stuck pipe usually makes the sticking force larger. The filter cake that protects the formation from lost mud is the same cake the pipe sticks to.