Books like DraftKings and FanDuel get their lines from market makers like Circa. Market makers use a variety of information for setting initial lines (you'll have to go ask them), but one of the main ways they move lines afterwards is professional action. That is, if some person or entity who Circa knows to be a profitable professional better puts down a large bet in a certain direction, Circa will move their line in that direction.

Where did that entity get that information, and how are they right so often? Your guess is as good as mine. I'm not accusing anyone of anything.