Discovered that if you place three lines of tracks beside each other facing North/South, the minecarts placed on them will push each other and travel a short distance without any external force. Video is attached for better explanation.

What I expected to happen was:

Carts remain stationary.

What actually happened was:

Two of the carts will move a short distance in opposite directions. If there are blocks blocking them, they'll travel a much longer distance together.

Steps to Reproduce:

1. Face North/South.

2. Build three separate lines of rails beside each other

3. Place carts on the rails, carts will move once the third is in place.

4. Remove carts, place blocks at the end of the three rails.

5. Place carts against the blocks.

6. Carts will move a much longer distance.