One possibility is that after power break, the OpenSprinkler booted much faster than the router. The firmware uses the following logic to check whether it should go into Ethernet mode or WiFi mode: 1) it detects if the Ethernet module exists by sending a command to the Ethernet module and verify the return value; 2) if it exists, it then does DHCP to attempt to obtain a valid IP address — this step has a 60 seconds time out, if it isn’t able to get valid DHCP within 60 seconds, it goes into WiFi mode. So this could explain why it went into WiFi.
A work-around is to set a static IP on your OpenSprinkler — when using static IP, it will bypass DHCP so as long as it passes step 1, it will stay in wired Ethernet mode. However, setting a static IP on the controller has its own risks: it might result in a IP conflict if your router ends up assigning the same IP to another device. You can avoid that by changing your router’s DHCP configuration to limit the range of DHCP IPs, and give your OpenSprinkler an IP outside of that range.
On the other hand, if this is a one-off issue and you do not anticipate the power break to happen often, perhaps just leave it as is.