Could it be that an extremely narrow hairline crack that would not typically be noticed using regular media are showing up as the little points clean into the hairline crack?
Perhaps there are (more-frequently than we are aware of) extremely tiny cracks in the case mouths which mostly go undetected using regular media but are suddenly brought into view with the pins.
I also wanted to mention that I do lube the necks prior to resizing. I use RCBS case lube on a Q-tip, but still find the expander is more difficult to remove from the cases than when I was tumbling with corn (I used the same lube for corn).