How it came to pass is unclear in post #1, but the photograph is pretty clear why he is trying to seat it now. The primer prevents the cartridge from coming out of the shell holder.
I speculate that Sigcurious was loading in a Turret Press using the continuous mode, completed all the operations (except only partially completed the primer seating part) and discovered the problem, a bullet that would not come out of the shell holder because of a primer not fully seated.
Ahh...That makes sense then...
I do not load on a progressive, so it did not occur to me that all of the operations were going on simultaneously...
Would the short stroke not have affected all other operations also?
May he have a cartridge with an under resized case (loose neck), one with a short charge, and one with a soft crimp also then?
Never mind...I just reread where his setup primes on the down stroke...