It was most likely a desyncronization of your client and his client. This happens when one or both of you disconnect from the server for a moment. You Rain of Fired on your side, but on his side, you never played the spell, so the creatures didn't die. Then, on his turn, when his computer sent the information back to you through the server, it registered that the creatures were still there and it replaced them on his field.
You might want to check your internet connection. I've noticed that desyncs like this happen more for wireless connections (even those with excellent connection strength) because WLAN connections disconnect and reconnect often, usually too quick for us to notice, but if they happen at the right time, it could cause a desync.
