ORIGINAL: ChuckW
To me, it sounds like the spektrum receiver went into fail safe for some reason. Problem is, it sounds like the fail safe position was full throttle rather than idle. Was it bound with the throttle sick forward or was the throttle maybe reversed after binding? That happened to me on an electric when I accidentally turned the transmitter off. If that's the case though, why did it lose signal and go into fail safe? Maybe the antenna being so close to the ground is an issue?
What you said sounds like the problem and also someone setting down a transmitter with the plane unrestrained, is totally bonkers(asking for an accident).
What you also need to understand wheather it be a 2.4 system or 72 they are not surface radio systems, when you get them that close to the surface of ground they will do funning things. I have seen it with 72 also.