Could it be something to do with soul-tickling?

Sorry.

No, I don't think it happens, it's just an expression we use, like kicking the bucket or popping yer clogs.

There are some things that make toes curl - look up the babinski reflex for example. My toes curl when I hear fingernails dragged down a blackboard.

An undertaker would probably give you a definitive answer, but it's possible that changes in tension during rigor mortis would result in curled toes. If, say, one group of muscles stiffened slightly more quickly than another then there could be movement. There's also a phenomenon called postmortem spasm that can mean a corpse retains a posture from the moment of death e.g. clasping a handle tightly in the fist. This is often associated with a violent end so can be useful in forensic work.