thanks for all the advice! I think that may be the answer. I wasn't paying too close attention to when the pins were sliding, but I'm pretty sure that it was happening when I inserted them from the outside towards the center.
I switched to that direction because it was a little easier to get it started in the holes to get everything lined up.
Next time I tow next week, I'll try to go from the center out and see if that solves it for me. That makes sense, because that is the direction that the pins shifted towards the outside.
thanks all! that was a quick and easy answer.