A lower rated player has less pressure/expectations of winning and is possibly more relaxed in their thinking on moves as well as being more careful knowing their opponent is much better. Couple this with a higher rated player taking
an opinion of thinking they should win easily, not playing maybe as well as they can and not thinking too hard on each move because they expect to win with little effort. This can be fatal and is probably the reason why half the games won by
lower rated players than their opponents happen - Chess.com forum
Games from a Class C tournament in which the top seed didn't get a look in
Diagram - How does Black capitalise on his position with his
next move ?