It may have stalled. It may have been a control surface incorrectly mixed or a elevon/aileron servo wasn't connected which caused the roll. So you have to repair and fly it again to see! [8D]

I'm sure you'll ensure all control surfaces are neutral before the next flight, like you did the first time, and are functioning properly - Up/down elevator give exactly the same elevon throw.

At that point, as the others with experience with these models have said, it needs some speed to become stable and for the powerplant to do it's thing, so a good hefty level throw into a slight headwind seems to be the best approach. Or, you could throw it off a steep cliff :-) Just Kidding! It's a joke. But you would get nice initial airspeed.

Good luck and I hope you come back and tell us it flew successfully!