My explanation is Murphy's Law. You all know that this means "the worst will always happen". However, QED mounted an investigation of this quite a few years ago.

[Parenthesis: For those that don't know, QED was a BBC TV series that looked into a variety of scientific questions, attempting to assert some kind of empirical proof for or against. Originally, it was reasonably serious in its approach. In later years, which is when I remember the "Murphy's Law" episode dating from, it became rather "pop" & a bit superficial.]

Its conclusion was that Murphy's Law is a matter of perception, rather than statistics.

The bread does not always fall butter side down.

The other queue does not always move quicker.

Particularly with regard to the latter, you do not remember the occasions when your queue moved quicker, you do not remember the occasions when your queue moved slower but you were distracted (a phone call, a conversation with a friend...), you do not really remember the times when you weren't paying much attention to how fast your queue was moving.

What you remember is the times when you were in a hurry and your queue was moving slower. Each & every occurence reinforces the perception that "The other queue always moves quicker", "the bread always falls butter side down", etc.

It's not that it does, it's just that you remember when it does, because it's such a nuisance when it does! I suspect the same perception is true of Macbeth. Jeanie doesn't remember the time that she got a part in a play, & something nice happened to her. She remembers the time she got a part in a play & something nasty happened to her.

AND THAT PLAY WAS MACBETH & IT HAS THE REPUTATION OF BEING AN UNLUCKY PLAY!

QED, really...