Being on the bleeding edge is great and all, but there are drawbacks. For example, many users of Android 5.0 devices are reporting that apps seem to be restarting an awful lot in the background. There are also performance issues that seem to crop up more the longer a device runs without being restarted.
Android is supposed to manage background processes all by itself. When you leave an app, it remains in the background until you return to it or the system needs memory and closes it. In Lollipop, users report even apps with persistent notifications are being closed sometimes. This could be a sign of memory management issues or memory leaks. It could even just be a bug that's closing apps too aggressively, even when there's plenty of RAM.
This doesn't seem to happen to all phones and tablets on Lollipop, though. I can recall specific instances on the Shield Tablet running Lollipop that games have remained active in the background for several hours. I feel like the Nexus 9 does suffer from this bug though. Nexus 5 users are also quite vocal. There's a Google code page with a good number of responses, so hopefully someone at Google is working on 5.0.1.