Winfield Scott is locked and impossible to use in any fashion for the first year of the war and then he resigns. The rollover tip notes that he will only activate if attacked. I'm assuming this is working exactly as designed and, in fact, matches history where he was disabled at length because of war injuries and never really got back into action.
Other units randomly get de-activated and I'm not sure why or for how long. Is it by design that we're left with no knowledge of exactly when they can return to being active? Does de-activation occur for specific reasons, like major loss of cohesion, big battle losses, or simply random events?