Originally Posted by cubewheels

The hub will spin backward by itself if the chain is about to come off any of the crank ring from the bottom.

For the chain to come off from the bottom of the big ring and straight to the frame, skipping the small ring, then you are somehow or something is pushing the chain sideways towards the frame at the bottom while you move the bike around.

By any chance you're carrying the bike with the drive side facing you? Your legs could be pushing the chain in without you noticing it. OR something on the ground is coming into contact with the chain as you move the bike around.

The only time the chain can come off by its own is due to "cross chaining" if the chain is on the big ring and also the biggest sprocket at the back (in terms of physical size). But since you mention the smallest rear ring, it is really weird the chain is getting to the frame instead.