Why do people search for a match, then decline it, so often?

Seriously, the only reason I can think of for someone actually clicking decline match when it comes up is if something comes up that they suddenly have to do instead of play. But this seems to happen way too much for it to really be that.

And I'm not talking about it just timing out, I mean when you barely see the Accept/Decline box before someone has declined it.

Can anyone think of a single other reason?