Does the air move from the center vents to the dash/defroster vents? Sounds like a vacuum situation.
Low vacuum = Climbing hill
High vacuum = Decel down the hill
Vacuum is what controls the vent air position, if there is a fault in it, it will default to the defrost position. So if you have a bad vacuum reservoir or something (not 100% familiar with ford's system), it may be fine all day long at normal driving conditions. But if you start to climb a hill, that vacuum may run low moving them up to defrost.