Probably primers sealed from the factory, I have seen the stuff sort of act like rubber stretching...a real pain to decap.
The RCBS universal decapping die helps with these because it is also spring loaded....problem is, spent primers all over the place because of the way it pops them out.I've run across some that were hard ot get out because of the crimp. The decapping rod actually rounded out the primer instead of popping it all the way out.
Yep, I always decap first, and the Lee Decapping Die does work well...that is what Coal was doing when he ran into this issue too...About the only time I hear this complaint is with Dillon sizing dies.
I recap with a Lee Universal Decapping Die for about 25 years without any issue. I inspect the case, deprime, and sort into correct bin by cartridge.