Some Lollipop owners are experiencing some memory leak problems with their devices. Android OS 5.0.1 apparently shuts down apps and constant home screen refreshes. While the Android community has been waiting on Lollipop to come to their devices, it is apparent that there are still a number of bugs to be squashed and that's possibly why it is taking so long to roll out the update.
According to Phandroid, "The thread on Google’s Android Issue Tracker shows a range of Nexus devices afflicted, everything from Nexus 7 2013, to Nexus 4, and Nexus 5 showing Android system eating upwards of 1.2GB+ of RAM (normal usage is around 500MB)." The problem is noted and will likely get a fix soon.
Source: Phandroid