Reasons for a vServer's reboot can include maintenance at the host node. 
In rare cases migration of a virtual server to another host node can cause a short offline time.

After finishing the maintenance at the host node all virtual servers are rebooted automatically after starting the host node.

Please pay heed to an activated Autostart mode.
Whether the Autostart mode is activated or not can be seen in the customer center after selecting the contract under "Start/Stop". Here you can also enable the Autostart mode.