Steps to reproduce:

1. Import and open the attached world: MC-275593.zip
2. Activate the impulse command block (it will kill all of the existing minecarts)
3. Wait until the minecarts start stacking
   Some of them move very fast (nearly instantaneously) towards the "end" of the stack
4. Break the lowest block of the pillar towards which the minecarts are stacking
   Some of the minecarts spread out towards the direction of the pillar very quickly (nearly instantaneously)

Original description:
when you put minecarts on a staircase made of blocks, and on top of it are powered rails,set the minecart max speed to 1000, the minecarts start stacking on top of each others, and if you make a wall one block further than the first minecart is, and there's enough of minecarts, some invisible force starts pulling the minecarts towards the wall, and if you break the wall the minecarts will start flying to the direction of the wall, it looks like there is a gravity field somewhere.