Etiquette.

Where I'm from, if you make plans with someone and decline to keep them, it's called snubbing. If the snubbing was accidental (i.e., your plans changed, but you forgot to call), then one typically apologizes and explains the oversight.

Doing this once or twice can be forgiven as absentmindedness, doing it three or more times with absolute politeness begins to be insulting. I'm giving Mr. Swing one more chance before I get angry with him for insulting my husband. (He's only "snubbed" me once, so far, so getting angry on my own behalf is not called for yet.)

As my husband knows, I am dangerous when defending or standing up for one of my own. If you know him, have him tell you the story of when someone got ahold of his password for his telephone messages in college and locked him out of it while his sister was in the hospital.