A few random thoughts with a few assumptions
Are you by chance crimping? Have the cases been trimmed at all? Is the bulge uniform around the shoulder? What brand of dies?
I suspect that possibly the seating die might be a little deep and some of the longer cases are hitting the crimp, causing the shoulder to bulge.
Resize a few cases and test chamber them before loading to eliminate the sizing operation from the cause. Run the unloaded cases through the seating die and test chamber again to test if your inadvertently crimping the longer cases.
Verify the seating die is correctly setup, Empty case, Top of stroke, Screw Die down till you feel the crimp, Back off 0.5-1 turn, lock and the use the screw / nut on the top to set seating depth.
If the case is too deep in the sizing die you generally dimple the shoulder or concertina the neck rather then bulge a shoulder. If case length varies too much and the seating die / crimp is setup on a short case then when you crimp a longer case it'll bulge the shoulder (I've done this and had to scrap 300 cases
).
Brett