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 ?
* Tournament games roo62 won with a score of 5/6