This works only with Minecart Experiments enabled.

Steps to reproduce:

1. Build a rail track (I used powered rails) that ends in front of a line of soul sand. You can use the video behavior2.mp4 as guidance. Build this facing north (so the minecarts travel in the north direction).
2. Summon a lot of minecarts on one rail (using a command block).
3. Push them in the direction of the soul sand.
4. As soon as the minecart cluster collides with the soul sand, its rotation changes from 270 to 90 degrees, producing a twitching effect (which is not very well visible in the video).

The other three directions this can be built in exhibit normal behavior where the minecarts do not change rotation.