"come a cropper" ... the intention of the parties was not realized. How does this relate?

Methinks you got the gist of your law professor's meaning, without the specifics. He meant things didn't work out because of some misfortune, as we can see from this history:

"We use come a cropper now to mean that a person has been struck by some serious misfortune, but it derives from hunting, where it originally meant a heavy fall from a horse. Its first appearance was in 1858, in a late and undistinguished work called Ask Mamma, by that well-known Victorian writer on hunting, R S Surtees, who’s perhaps best known for Jorrock’s Jaunts and Jollities."