Android 5.1 made some major changes to the way recent apps are handled in Android. You can include Chrome tabs in the recent app list, and apps that are called up from another activity get their own card to keep things more consistent. However, Google also decided to never clear the list of recent app cards. It just goes on and on and on... it's kind of annoying. "Recently" from Chainfire fixes that, but you need root.

Chainfire's New App 'Recently' Tells The Never-Ending List Of Recent Apps In Lollipop To Take A Hike [Root Only] was written by the awesome team at Android Police.