make money if bettors are attracted to your lines because they don't reflect probabilities. If bettors are betting against the sports book, because the sports book's line was ridiculous, the sports book has to pay money to all those winning bettors and it goes broke. It makes money by being right most of the time.
Bookmakers can only make money if most of their lines accurately predict the outcome of the game. If everyone jumps on the BYU-AZ line and bets BYU to win by four points or more, and BYU then wins by four points or more, the bookmakers absorb a heavy loss, which they don't like, obviously. Vegas bookmakers still believe BYU's offense isn't good enough to win by a big margin over Arizona; hence the current line. If heavy betting is coming in all on one side, the sports books move their lines in recognition of the fact that they're attacting TOO MANY bets on one side only. Too many bets on one side is dangerous to to sports books. It's just like a bank. If everyone is withdrawing money at once, you're getting a lot of business, but it's the kind of business that puts you out of business. You need deposits to balance out the withdrawals, and in betting, you only want to attract a lot of bets if it's sufficiently balanced.