For me the salient points are:
*You had a suspicion at the time that the man reading the paper had something to do with it (and the phenomenon finished after he had hastily left). Our first hunches are often correct.
* The `foot` was not behaving like an ordinary foot - but wriggling about like `a dog on a leash` (i.e drawing attention to itself).
* It was sufficiently physical to cause someone - a woman - to move their own foot away from it.
* This happened on October 31st - Halloween, a day which I understand Americans take really quite seriously and which is a time of general pranksomeness and tricks.
* Foot puppets are a thing:
So - as a provisional solution - the details need fleshing out (no pun!) - I am going to propose that the gentleman with the newspaper instituted some sort of seasonal prank using his own foot disguised as a woman's foot.
I have thought this too but he would have needed to have three legs, because I looked under the seat as it was happening and I could see both his feet in black shoes as the weird leg was moving about. And he was also sitting upright. To be able to reach under the seat in front you would have to slide down on to your lower back to get your leg under the seat. Or have an incredibly long leg.
Also, if someone was moving their foot at the speed it was going you would have body movement. His upper body was not moving.