Social incompetence - For example, my DH saw my BIL whilst out and about, as DH went to walk past he said hello, BIL stared at him like a rabbit in the headlights, hot heeled and walked in the opposite direction leaving DH wondering what he'd done wrong and why BIL couldnt have just said hello.
I have seen this many times with people, they look right at you, they know you and do this instead of just saying hello. My Mum and Dad did it to one of my employees who had fixed both their cars for years, My brother did it a few times to my stepson at the gym until my step son just started ignoring him.
Its just so awkward and unnecessary especially if you actually know the person saying hello, have seen them and can't avoid them. Why would anyone do this? Saying hello back is all that is needed, no-one needs to become best mates.
(No history by the way, all completely unexplained, which is why I find it so puzzling and so rude)