I hate to generalize (no, I don't), but this is a common trait for females to wait for the last second, then give the directions in a non-commanding way. Like "You might have been thinking of getting in the left lane back there, because we just passed the intersection we should have turned at...."
I have been training my wife to give definite instructions prior to the action point. It's a work in process. It usually works like this: Me: Left lane or right lane? Her: We don't turn here. Me: Yeah, explective, I know we don't turn here, but we are explective turning soon, so, which explective lane should I be getting in before the explective turn? Explectives are bad, but it's the only way to get her attention.
It's a work in process.
As to the nagging, she was both wrong and an idiot, so the nagging was her attempt to place blame for her own inadequacies on you.