Bullet length and case neck splitting

Hello again. I have a 6.5x57 Mauser bolt rifle that I only occasionally shoot. I had been handloading for it some years ago. I was successfully shooting 140 grain Sierra Match King #1470 using 33 gr. of IMR4064, OAL of 3.06" with no problems.
Then I thought, why use a heavy more expensive bullet if I could switch to something lighter with a bit less recoil and a little less expensive. So I tried the 85 grain Sierra #1700 with 38.2gr of IMR3031, OAL of 2.9".
The problem that developed is that with the shorter, lighter bullet, I began to experience case neck splits. So my question is, why would the lighter bullet cause the case neck splitting? Could there be a chamber problem? Not all cases split. I have been using 7mm Mauser cases necked down to 6.5 using the sizing die. This is new brass, fired only once before to fire form the cases. I have stopped using the 85 grain bullet as I don't want to ruin more cases. But I'm curious about the splitting.
Thanks.

Ok, sounds good. Thank you.

Thanks for the help on this. Random, yes, same lot I had used with the heavier bullet.
I'm surprised that they split already. I keep a record of each group and this was new brass that had been fired only once or twice before. But I will definitely anneal what I have that's not loaded. I have 70 of them that are loaded. I'm not sure if I should just shoot them as is and hope for the best, or disassemble them and anneal. I'd have to get the primers out too, I would think. Lots of work. But I am retired, so I have lots of time.
Should new brass be annealed too?