So what are the causes, what makes people act like that? I've found that even very nice people can end up doing that on some occasions. Ignoring completely innocent texts, online messages, etc. I'd be interested in all kinds of causes, reasons, description of dynamics .... Not just the most common reasons, but anything really that you can think of.

Why I'm personally interested: I'm at the point in my life where I am seriously considering seeing this behaviour as completely unacceptable and extremely rude, like I'm willing to forever cut anyone out of my life who tries to pull that off on me. I am THAT fed up with it.