Mobs (such as villagers) that are riding in a minecart, are able to control the minecart and move it along rail lines. This was a bug in java edition, but was fixed in 1.13, and should be fixed on bedrock edition as well.
How to reproduce:
1. Set up a large circle of basic rails.
2. Get a villager into a minecart on those rails.
3. Make sure the cart is not moving at all.
4. The villager should start to move the minecart on its own shortly.
5. If it doesn't, spawn a zombie nearby and the villager should roll on away. Of course make sure to kill the zombie before it attacks the villager, or nudges the cart
You can see video footage of this bug in action, along with how to reproduce here: