For me, it seems it was an Xposed framework extension called HowGiveLolli. Definitely non-essential so I can leave it disabled until the author issues a fix. The reboots, for me, were anywhere from 5min to 12hr apart, it's been 16hr since the last one (when I disabled the plugin) so, at least for me this seems fixed.
I'd say in most cases this is a software issue, especially given the known memory leak in Lollipop; certain apps likely trigger the leak worse than others, and even more so given certain configuration options (e.g. synching more frequently). The easiest way to track it down (unfurtunately, it's a bit time-consuming) is to disable or uninstall any non-critical apps; if that doesn't stop the rebooting, disable or uninstall everything but the Settings app (otherwise you won't be able to re-enable or re-install things). If that doesn't stop the reboots, you've likely got a hardware issue; if it does, re-enable or re-install your apps, one at a time (probably the most critical apps first, so you can start actually using your phone sooner) until the reboots start again. When they start, you've found the culprit; clear its cache and data and if that doesn't fix the reboots uninstall or disable it. Keep in mind that you may have multiple apps causing reboots, so don't get excited and reinstall or re-enable ALL of your apps when you've found one that causes issues; keep with the one-by-one program until you've checked them all.
Hope this helps, and good luck!