When minecarts pick up entities, their motion is changed incorrectly. Instead of continuing in the same direction it approached the entity, it will either go the opposite direction or come to a halt. It appears that when a minecart picks up an entity every second block after it last hit a wall or changes direction, it will come to a complete halt. Otherwise, it will go the opposite direction (back the way it approached the entity).
From extensive testing, I can conclude that this bug is not directional. A picture is provided for a better visual explanation, with the stained clay showing the motion of the minecart after picking up an entity on the adjacent rail, with the minecart approaching the entity in the direction shown by the comparators.