They become angels of death. They managed to kill Madrid and Shaktar. I don't think it's luck, but thanks to hard work. But it could also be because Madrid and Shaktar underestimate him too much.
I don't think Real Madrid and Shakhtar got loses because they underestimate Sheriff. Sheriff won because they played well and be confident without any pressure. Sometimes, once you play nothing to lose, you can optimize the chance to win. While those teams that play with too high expectation, can be defeated since losing focus and control during the match. That's probably happening to Real Madrid and Shakhtar.