I do tend to close it after I'm done playing, so it's not that. It might also be a coincidence; I'm doing an experiment today where I barely use my phone off of wifi for anything else to see how it goes.
One thing EA did in the past several months that I really appreciate is not auto-downloading updates when you're off of wifi and open the game. There's a store across the street from my office that I can go to and get free wi-fi for when there is an in-game update to download, and it's nice that I don't have to frantically close the app to avoid using lots of data downloading it...