Could be due to powder deterioration.
The shooting community is ignorant of the fact that gunpowder has a shelf life and as powder deteriorates it causes more problems than it cures.
As gunpowder breaks down it releases NOx, this reacting with the water in humidity, one by product is NO2 acid gas, and this will crack brass and leave green corrosion spots inside the brass and on copper jackets.
Pull a couple of bullets and see if you have evidence of tiny green spots, like found on these US military bullets.
If so, pull the bullets and dump the powder. Old gunpowder can, has, and will blow up firearms, this is all due to burn rate instability. Some of the cases may be salvageable, but you will experience case splits on some or all of them.