Given that when you backed off and they were more snug, I'll guess too much downward taper crimp pressure. As to why, I can only guess that too much downward force on the mouth causes the case to "squat" down and widen the area just below it. Never had that happen, so I can only visualize it in my head. The taper crimp die has a fairly linear taper inside. Once it has driven the mouth down into the groove as far as the pitch of the taper will allow, continuing to push the case deeper into the die will grab the portion of the case below the groove and try to drive it into the band or body. Trying to drive relatively hard brass into relatively solid copper is going to in all likelihood, seize up the case against the die wall. Further pushing will as I said, drive the entire case downward.
Imparting the taper into either a groove or to a lesser degree, a cannelure, should present minimal resistance to the handle stroke, so a good indicator of too great a travel would be when the handle resistance goes up noticeably in a short period of time. In the case of smooth walled bullets, the taper crimping cycle is immediately harder to impart. Jacketed lead core bullet walls yield easier than solid copper, so you can impart a taper crimp even on the smooth walled bullets without needing so much force as to thin the case mouth walls. Experience has taught me that the benefit of the taper crimp is still hard to appreciate on smooth sided bullets. They have always for me, been more challenging to crimp. I sure wish the Hornady 250gr FTX and 300gr SST-ML bullets had a cannelure right at the optimum seating depth for the AR magazine. It would make the results more consistent from session to session. Luckily, white tails don't need such heavy bullets in this caliber, to reliably drop them in their tracks.
I do not run my taper die squeaky clean inside. While it is not dripping lube, I do like to keep the inside wall somewhat lubed with a little and I mean little bit of gun oil applied with a long, double ended q-tip. If upon applying the light layer of oil, the q-tip comes out dirty, I flip it to the clean end and wipe it out, then repeat with new ones until they come out clean. However many passes that takes, once the applied lube end of the q-tip comes out clean, I flip and wipe one last time with the dry end. I'll still leave a fine coating on the inside, but not enough to worry about it contaminating the case or bullet. That having been said, I still wipe down my finished loads with a clean rag ever so lightly moistened with denatured alcohol. Emphasis on ever so lightly. I lay the clean, moistened rag on the upturned palm of my hand, lay three finished round on it, flip the side of the rag up on top of them and roll them a pass or two between my hands. Easier seen than written.
One last mention. When I say q-tip, I don't mean the ones for your ears. Technically, what I use are double ended, cotton tipped applicators. They have a wood shaft and are about 6 inches long. You can get them from countless sources for around $12.50 per box of 500.
ExampleA box lasts me about a year.
"It's a clean machine..." Hoot