It's challenging to determine the exact reason behind someone's actions without more context or information about the individual. People do various things for a wide range of reasons, and it's often challenging to interpret their motivations accurately based on a single observation.
However, here are some possible reasons why the guy you know might have pulled his hood down as you were passing him in the rain:
Politeness: He might have wanted to acknowledge your presence or show respect by uncovering his head as you passed by.
Social norms: In some cultures or social settings, it is considered polite to remove one's hat or hood when greeting someone or when passing by someone they know.
Comfort: Perhaps he found it more comfortable to lower his hood momentarily, either due to rain dripping into his face or for some other reason.
Accidental: It could have been coincidental, and he may not have even noticed your presence or the timing of his action.
Random behavior: Sometimes, people do things without any specific reason, especially if they are lost in their thoughts or preoccupied with something else.
Playful or teasing gesture: If you have a friendly or playful relationship, he might have pulled his hood down as a joke or a playful gesture.
Ultimately, without more information or context, it's challenging to pinpoint the exact reason for his actions. If you have a close relationship with this person, you might consider asking him casually about it if you feel comfortable doing so. However, if it was just a passing moment and you are unsure of the reason, it might be best to let it go as one of those unexplained occurrences in daily life.