Hi,
This is my first post here. I'm a lifelong journalist and thought I might be able to help with some of the more obscure slang words and phrases.
In my experience, "piling on" is a term taken from American football. It helps to know that in this sport, a player can score by carrying the ball across his opponent's goal line. But of course, the opponents try to stop him from doing so -- which involves "tackling" him (wrapping their arms around him and knocking him to the ground).
When a player is down, the play is over, so there's no need for other opposing players to fall on top of him, too. But if they do, it's called "piling on," and it's generally considered unsportsmanlike.
"Piling on" can also be applied to everyday situations in which several people attack one person, usually unfairly or unnecessarily.
For example: "It was bad enough when the boss yelled at me during our staff meeting, but then my co-workers started piling on."
So ... "rub it in" is close in meaning.
By the way, "pile in" is a different expression that usually refers to a group of people crowding into a vehicle. Example: "Come on, kids, pile in the car and I'll take you to Dairy Queen for ice cream."
I hope that helps.