It's got something to do with the end point of lines. I tried making thick lines by drawing them all straight. Failed. Then I drew diagonal lines between two lines after that, and the rider went whoooosh!
fil is pretty much correct. Each line that is touching has a driving force on the object (wheels). Due to the fact multiple lines can lay on top of each other, multiple drive forces occur.
I also have proof: Watch this code -18 1i 18 1i,u 1i -18 1i u 1i,k 1i -18 1i k 1i,a 1i -18 1i a 1i,0 1i -18 1i 0 1i,-a 1i -18 1i -k 1i,-u 1i -18 1i -u 1i## I laid a bunch of straight lines on top of the start