There really isn't any way to tell why a Portserver II rebooted.
A couple of things to do before calling for an RMA:
1. If you have another Portserver II, try the power supply from that unit with the rebooting unit. If its a power supply problem, this step will identify it.
2. Make sure the Portserver II has the latest "useable" version of firmware loaded on it. Some of the earlier versions did have bugs which caused a reboot under various conditions. By "useable" I'm referring to the fact that there are different hardware revisions and amounts of RAM installed on the Portserver II over the years. Please refer to the firmware page for details on how you can determine which version you can use: