I have seen such cracking in only 2 situations:
1) Cases that had been resized over and over again.
2) Very ancient milsurp cases that had been rotted by the powder.
How old were the rounds that you fired? Were they, maybe, stored at above-average temperatures. That, at any time in the past, could have initiated chemical degeneration and internal corrosion of the brass.
Is your neck sizer really only sizing the neck? The neck should be lightly lubricated, and after sizing you should see a line just above the start of the neck where the lube has been pushed back.
Swedish Mausers have chambers a touch longer than a SAAMI gauge would approve of, and are sometimes mistakenly thought to have excessive headspace when they are, in fact, in perfect shape.
Do the "lubeline" check. You may need to back off the necksizing die about 1/64". But in spite of that, I think you have a dud lot of ammo.
Patrick