Every time I search reloading data for a magnum caliber I always come across the crimping conversation and bullet jump.Partial burn will do that
Third option - low bullet tension from a bad or no crimp.
Magnum powders need a lot of pressure to burn properly so if the bullet releases and jumps the gap before the pressure gets high enough the burn extinguishes at the cylinder gap.
Had a similar problem in 38spl with thin walled brass and slightly undersized plated bullets.
Change to thick walled brass - no issues
357 brass - no issues (firing in a 357 the 38 case created a 0.1" jump)
Cast bullet the same weight - no issues
With 500 magnum I'd assume the recoil would easily start a weakly crimped bullet moving forward reducing initial pressure even further.
I haven't experienced it, but I read enough about it when I started reloading that I make sure my bullets have proper crimp. I did check them in the beginning (shooting 3 rounds then looking at the last two).