gjw:
I think the previous two posters have it right. There IS a puncture in the cup... or there would be no leakage. You may not be able to see it, but it's there.

Solution, try different primers if you reload. Federals or Remington seem less apt to pierce. If you buy ammunition, try a different brand.

I have never been a fan of Cheddite primers.

Best, Kensal