This sounds like a case of too much pressure from any of the reasons sighted above. As long as the round fired you should safely be able to tap the bolt open. I would pull a sample of your remaining rounds and check the amount of powder (was it the right powder?) and case length. Another question for you. Did you have any trouble chambering the round? It could have been that your sizing die backed out without you noticing. This and too long a case would result in difficulity closing the bolt.
I know you have been loading for 8 years as you said but some safety tips:
Always keep a clean loading bench.
Perform one operation at a time. (sizing, trimming, priming, etc.)
Always refer to the reloading manual. Don't try to remember loads.
Only keep the components on the bench needed for the cartridge you are loading. (one powder, one bullet, etc.)
Don't get distracted.
Personally, depending on cartridge being loaded, I weigh every charge. For lower pressure cartridges and pistol cartridges, I set my powder thrower and weigh every 10th charge. That way, if I have a concern I only have to dump 10 charges.

Hope you find the problem and you get your bolt open without damage.