Why does the A/C change modes, from panel to defrost all on it's own? on 2003 Mercury Grand Marquis

At idle A/C blows through the panel. As I roll down the road the A/C will start blowing out the defrost all on it's own. WHY?!?!?

Asked by for the 2003 Mercury Grand Marquis
loss of vac to controller. ck vac lines under hood for cracks or breaks. if there ok ck under dash if unable to find have ssmoke test done to determine leaks