While searching to find you pictures, I came across this forum post
https://ogrforum.ogaugerr.com/...1#153463017335878531
This photo by @Ray of sunshine while intended to show a different problem is semi-relevant
You can see how both trucks are "twisting" compared to the straight track and the couplers are not in a straight line parallel with the track. Now imagine this same setup on a curve and the couplers shift and are trying to push the truck right off the track. Fixed couplers attached to the truck that follow and "steer" the truck (or the truck attempts to steer them) VS flexible coupler to truck connection.