Yup. I tend to make eye contact or wave or nod toward every person I pass on the trail--whether said person is walking, riding, jogging...whatever. It's just good manners to acknowledge other people when passing face to face in close quarters.
I pass thousands of strangers in close quarters every day. Rather than say hi to every one of them, I say hi to none of them. I guess that makes me rude.
Then when I'm in a situation where there aren't quite so many people, the practice carries over. It's just a habit. Nothing more sinister (or rude) than that.
I'm curious as to how many of the stranger-acknowledgers live in big cities, medium sized town, small towns, or what.