Could be any number of things......brass, primer, headspace, firing pin or a combination of some sort.
I'm guessing from the info you provided that these are your reloads. So, from that let's check the fairly easy stuff first assuming there was a mark on the primer (just not very deep).
1. Is this the only time it's happened?
2. New or old primers? What brand?
3. Check depth of primer pocket on that pc. versus the others
4. If you have the tools, check the case head to datum length on that brass vs. the others
That should give you a very good start!
Bookmarks