How would the hawk trapped in a cage know the flood was over. Actually an uninjured hawk would have just flown away from the flooded areas.
IMO the reason for the hawk's actions are pain and lack of food. He ended up in the car as the first place out of the wind he could find. He was unhappy around people (based on the intensity of his glare at everyone) but wasn't able to leave. Notice in the video from the shelter that as he recovered and was fed, he got nastier.