In this sentence "Pile on" is used as a phrasal verb, it means - to join other people in criticizing something or someone in usually an unfair way.

In American football, when the ball is dropped, nearby players will jump towards it to try to grab it for their team. Sometimes, this causes almost all of the players on the field to jump on top of each other, with all of them trying to grab the ball. Here's a picture: https://c8.alamy.com/comp/KPBP04/us-army-and-us-navy-football-players-pile-on-top-of-each-other-during-KPBP04.jpg We often use this metaphorically. If you say something, and five people criticize you, and then a sixth person also criticizes you, he is "piling on": he is adding unnecessary extra criticism after after all of the useful citicism has already been given.

'sorry to pile on' in this context means 'sorry to be so negative about it' or 'Sorry to be so critical'. "Sorry to be no negative about it but this movie ...." "Sorry I'm being so critical, but this movie ..."

If you put pieces of paper in a stack, it could be called a pile. If you add more paper to the stack then you would "pile on" more paper. So in this case the writer is being apologetic for having nothing good to say about the movie. They are piling on criticisms.