I'd do 2 things, 1st ones easy, take 4 or 5 of those rounds and push them against some wood, like a work table, at the bullet nose, push hard but only with hand pressure and see if the bullets move, if they do, bad crimp from the factory. If you only have 50, hell push them all.
2nd, remove the barrel and try the bullet fit in the chamber, should go very easy, sit flush, and come out easy.
Another thing would be to examine those rounds that did push back under a magnifying glass and see if there any obvious impact marks, at the nose, and around the sides at the ogive. If there are even marks around the sides, there may not be any lead cut into the rifling, and the rifling could be catching the bullet and pushing it back. Since this happening on loading, I'm betting you can push the bullets in by hand.