When the rear der allows the chain to move past the large cog it get's wedged between the cog and the guard/spokes. If the der isn't well adjusted or has been bent inward then the large cog limit screw is likely not set correct to limit the chain's travels. The other common way the chain derails when the bike is set down is that gravity tugs on the chain's length and combined with a back pedal can lead to "backwards" shifting. Picking up the rear wheel off the ground and pedaling a stroke or three usually get's the chain back on the cog it was last shifted onto. Andy