Bookmakers' odds are merely a reflection of how many people are willing to buy into a certain outcome at a particular price. That is to say that this shortening of Spurs' odds represents a large amount of betting activity on Spurs to win the league.
Of course, they also use complex algorithms to try to sway people into betting in ways that will be maximally profitable, and factor in a healthy overround so that they functionally never lose money no matter what the outcome.
So many people have an inaccurate perception of what bookies do and how they function. They make money by selling all odds at scam prices (i.e. shorter than their "real" value) rather than by accurately predicting events (which I think is what most people believe they do). There's no such thing as beating the bookies really.
(That and the fact that they're only semi-regulated, all operated out of tax havens, are exploitative as all hell.. Well, they're cnuts basically. Total cnuts. Don't gamble with bookies at all, friends.)