Hi, when you read rule 12.5 it says “Every player is entitled to occupy any position on the field not occupied by any opposing player, provided that they do not initiate contact in taking such a position, and are not moving in a reckless or dangerously aggressive manner.”
And then the annotations in 12.5 it is explained that “Moving in a ‘reckless manner’ could include running without looking where you are going for an extended period of time, or diving in a way that does not allow you to adjust to any legal changes of movement that an opponent might make.”
For me, following a player from behind so close that they cannot reduce their speed falls into the “Reckless manner” cattegory.