lines move when money comes in for the other side.
for example, if its +200 for benzema to arsenal and -200 for him to stay, and 60% of the money is on him coming to arsenal, they will lower the +200 to +150 to arsenal and -150 to stay, hoping that more people will put money on him staying at the better price
oddsmakers main goal is to get a near equal amount of money on both sides of a bet and profit off the small differences by being right more times than they are wrong