I was about to suggest if he may have been near a nest. We have Herring gulls that will dive bomb anything -including humans that get too close to their nest. Several years ago, gulls had built a nest between the 2 chimney stacks of the houses at the back of mine.
Once the chicks had reached a certain stage of maturity (they were starting to take an interest in outside of the nest) and briefly venturing on to the ridge tiles, the gulls became very protective of the surrounding area.
My next door neighbour used to wear a hard hat to go into his garden and even my cats avoided the bottom LH side of my garden or they would swoop. I've had them go for me in the past just for getting out of the car on my drive and walking to my front door. Those beasts are scary when they do swoop and believe me they have sharp beaks known to draw blood on many a time on people simply walking down the street. I'm certain Ive mentioned before on here about how territorial they can be and how even the postman refused to deliver mail to certain houses because of them. They are quite prone to calling for backup and before you know it, you may be victim of 6 or more gulls attacking you.
My older cat male remembers that year and is way more wary of them, but my slightly younger girl mustn't have experienced that particular year... as I think I may have also mentioned on here, when I was able, how I used to take both of my cats walkies on field. Zigs had been very poorly after a recent bad triaditis flare, and it was his first walkies out for a while and he must have got too near their area of another nest and a gull started dive bombing him. My tiny girl who is an absolute crap hunter went into full on alert mode and actually started having a go at the gull who was trying to attack her poorly brother. I was so surprised at how brave she was as she is such a smaller boned cat with zero attack skills leaping up in the air to counter attack the gull, whilst my bigger boned boy with excellent hunting & attack skills just ducked and cowered. I think I have a photo of it somewhere I'll try dig out if you like.