The reaction of a given diver to any emergency will run the gamut of possibilities ... everything from calm, by-the-book to completely irrational ... depending on the diver. But I think a lot of these suppositions are based on either stories told to you by an instructor, or things read on the internet.
Panic is a reaction induced by being faced with a problem you don't know how to resolve ... it's our basic, hard-wired "fight or flight" instinct. If a diver is approaching you for an air donation, they are acting on a resolution to their problem, and the chances of getting "mugged" are very unlikely. It's when that donation doesn't go well that their instincts are more likely to kick in ... and this is quite more likely with a new diver, who hasn't yet developed enough experience to think through alternatives than it is with a more experienced diver.
As for why your new diver wanted air, the most likely reason is that he just checked his gauge and realized he didn't have enough left to finish the dive, or make it to the surface. Or perhaps he was having a problem with his reg ... either it was breathing hard (improperly tuned or the adjustment knob on the spring was too tight) or maybe he just slurped some water and assumed his reg wasn't working right. Either of those possibilities would cause a diver to want air from someone else.
And FWIW - some divers put their primary on a necklace, and stow an octopus. Don't assume if you see a diver wearing a necklace that it's for the backup reg ... some, especially those who chronically dive in areas with surf, necklace their primary so if they lose it they can easily find and replace it.
... Bob (Grateful Diver)
