Word gets around, and people doing things that are inappropriate tends to get around faster. Obviously, you're not guaranteed to know about it every time that this happens. Sometimes you find out, sometimes you won't. It's when you do find out that you can try to do something about it (not that there's any change of getting the job back, but in terms of revenge on the people who got involved in your business).
Several times, I thought an interview went really well, and then they never got back to me. They don't give a reason.
Of course. That's typical, and usually the reason doesn't matter. It's rarely something scandalous that merits anything other but "eh, guess that didn't happen". Most of the time, getting turned down for a job is just a regular lack-of-chemistry thing not worth getting bent out of shape over.