Word for winning because the competitors were miraculously all worse

Actually, I think "win by default" IS often used to refer to this situation. Perhaps metaphorically: We didn't win because there was LITERALLY no competition, but because the competition that existed was so weak that it hardly counted. People will say "practically won by default" or "virtually won by default". But still, nothing in that phrase implies that you (or your team) was weak, just that the competition was very weak relative to you.

Barring that, I don't know of a phrase in that form that explicitly means, winning because the competition was so weak. I can't swear that there isn't one, but nothing comes to mind.

People say "it was an easy win" meaning you were much better than the competition. Again, though, doesn't necessarily indicate that your side was weak.

"The competition was weak".

That's all I can think of.

In Australia the phrase doing a Bradbury was born following Steven Bradbury's amazing Gold medal win in the speed skating at the 2002 Winter Olympics.

It denotes achieving an improbable victory through circumstances beyond one's control.

You can watch here https://www.youtube.com/watch?v=fAADWfJO2qM Basically, he was in last position for the whole race, except for the last quarter lap when all four skaters ahead of him fell, allowing him to coast through to the gold medal.

Probably the Idiom: by chance. (TFD)

Without plan; accidentally: They met by chance on a plane.

Possibly; perchance: Is he, by chance, her brother?

without advance planning

Synonyms: accidentally, circumstantially, unexpectedly

through chance, "To sleep, perchance to dream.."