NFL odds will move because the sportsbook is trying to balance their action on the game. When it comes to football odds, sportsbooks around the world make their money by taking out a small commission, commonly referred to as vigorish from every losing wager. It is because of this vigorish that the NFL move so much.
NFL are moved so that both sides of a game are attractive to bettors. For example, if one player bets on New England at -3, they risk the standard $110 to win $100 or whatever amount they choose based on the 11/10 ratio. Another player takes Pittsburgh at +3 in NFL for the same amount of $110 to win $100. The sportsbook makes $10 regardless of who wins the game as long as New England doesn’t win by exactly 3 points. The total amount wagered in this example is $220 dollars, so the profit for the sportsbook in this football odds example is about 4.5%.
What happens with NFL is that sportsbooks are rarely balanced equally on a game. When the action is not balanced the books have some things they can do. Let’s look at them. They can do absolutely nothing and hope that the game goes their way and that the side with the most money loses. They can move the NFL and try and attract more action on the other side and make it more expensive to lay points on the popular side. They can also do one other thing that most people don’t talk about but it happens all the time; and that is layoff their excess action at another sportsbook. Usually the sportsbooks will choose to move the football odds.
Some of the bigger books will let the NFL alone if they believe they have the right side of the game and that the money side is wrong. For example, if a sportsbook has $5,500 wagered on New England –3 in NFL odds and $2,200 wagered on Pittsburgh +3 in NFL odds, then the sportsbook stands to lose $2,800 if New England covers the NFL odds (They collect $2,200 from people who bet on Pittsburgh but have to pay $5,000 to players who bet on New England) but stands to win $3,500 if Pittsburgh covers (collects $5,500 from players who bet on New England but has to pay $2,000 to players who bet on Pittsburgh). This is how the NFL odds move everyday.
Football odds movement is an interesting study and one that you could spend many days looking at.