Probably because people are buying lots of packs to try and get a TOTW player. Packs come with around 3-4 contracts so if you have no need for more contracts then you can sell them for cheap to try and get some more coins to spend on packs and so on.
edit: oh for fucks sake ultimate team is going to give me a stroke. Up 2-1, no time left, haven't been able to complete a pass for ages, finally win back the ball and go to clear it. Oh wait, interception, soft shot gets saved but bounces up into the post, bounces once over the line and back out. 2-2 draw, whistle goes after i kick off. Fuck you Fifa
edit #2: Next game and again i can't seem to make a pass or know how to defend in the last 10 mins. brilliant