Or it could be any number of things.
He might be over-thinking the use of his new pistol
Or he could be having personal problems
Or he might not be sleeping well
Or he might be struggling with health issues
Or he might not acclimated himself to a different shooting platform
Or he might have expected a new gun to automatically improve his performance
Or he might have a pistol that needs different ammo
Or there might be something different about his diet
Or he might simply be in a temporary slump
It IS NOT easy to figure out when this happens. But it does, and with practice, analysis, using relaxation techniques, or simply step away from shooting for a while, it will come back
Or it won't
Like I said . . . . it's not easy