This bug arises when a minecart cluster (many minecarts clustered together in one block) derails off a rail track. This only happens when Minecart Experiments is disabled.

Steps to reproduce:

1. Build a rail track (I used powered rails). The direction this is built in is not important.
2. Summon a lot of minecarts on one rail (using a command block).
3. Push them in the direction of one end of the rail track.
4. As soon as the minecart cluster derails, it changes direction about 45 degrees and proceeds in the new direction.