Perhaps the hands that touched the baby seagull smelled of food.
I copied this excerpt from a person who studies bird behaviors............
In all my time of studying birds, there's one topic that always elicits a reaction—when I say birds can't smell or taste.
I'm not surprised that many people have an opinion on this subject. After watching birds select only certain types of seeds, fruit and suet from backyard feeders, it's difficult to believe birds don't rely on smell or taste to determine what to eat.
Even the experts can't agree, despite more than a century of research and debate. The results of studies in this area are often contradictory or simply inconclusive.
However, one fact is almost certain. Birds depend less on the senses of smell and taste than people do.
Most birds have little use for the sense of smell. The odors of food, prey, enemies or mates quickly disperse in the wind. Birds possess olfactory glands, but they're not well developed in most species, including the songbirds in our backyards.
The same is true for taste, which is related to smell. While humans have 9,000 taste buds, songbirds have fewer than 50.
That means the birds we feed around our homes must locate their food by sight or touch, two senses that are highly developed in birds.
The last sentence probably best explains what you witnessed. The other gulls probably thought that baby gull was given food and if you have seen gulls go after each other for food and I am sure most of us have they can be ruthless until the bird they are after gets away or drops or spits back up what it has. That baby gull obviously could not fend off attacks and the others persisted to get what they perceived the fellow fed him until it died.