I've long suspected that an element of an officer's drop in success may be promotion either above or away from one's talents.
Because he is good at, for example, divisional field command he eventually gets promoted to Corps and maybe even Army command where he either finds himself out of his depth, or bogged down with too many staff and administrative/logistical burdens.
Being talented at divisional field command does not mean the officer is similarly suited for higher command duties. The work that he excelled at is now being done by subordinates, who might not be as talented as he was in that job.
And this failure is reflected upwards.
This would also account for the replacement not being 'terminal' as Mr. Ricks put it.
Having an otherwise good officer fail at one job does not mean he's useless, simply employ him within his proven ability again, or find another area where he can apply his talents without doing harm.
In other words, you take a team that works, and then shuffle around the members job's. It isn't always a recipe for success.