Bug happens when you have two tracks sitting next to each other. Almost as if though the minecarts are too wide.
Place a minecart on each track try side by side, as if though you wanted them to sit idle next to each other. Upon doing this, one of the minecarts will shift and twist causing the other minecart to get pushed forward.
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This was actually the basis for minecart propulsion before powered rails in Java. It was called a "Minecart Booster."
The reason it happens is probably because the hitbox of the minecart is larger than a block, so it overlaps (and thus interacts with) a minecart on an adjacent rail.
This probably won't get changed, so do your best to wok around it.
This issue never existed until the 1.13 update. I know this because I, like so many others, had Redstone creations that utilized side by side idle minecarts. After the update these all started breaking and have been this way ever since.
Duplicate of MCPE-59302
After searching for an hour, I noticed someone posted the same issue a couple days ago.
https://bugs.mojang.com/browse/MCPE-59302