It looks like a bug slipped past the engineers at Google when they released the Android 5.1. The bug is ironic because it is one of the main reasons's the Android 5.1 update was created to begin with. It's a new memory leak. According to the report, Google is not only aware of the issue, they have supposedly already fixed it internally, but have yet to send out the fix. Google listed it as a “Future Release”, which likely means it will show up as an Android 5.1.1 or something similar.
We will keep you posted if we find out more. In the mean-time, feel free to let us know if you have experienced the memory leak issues that other folks have reported.
Source: Issue 159738 - android - Memory leak still present on android 5.1 - Android Open Source Project - Issue Tracker - Google Project Hosting