That's the definition of colluding, which is really the only reason a trade should be overturned.
Basically, I am guessing they are trying to gain an advantage on the rest of your league which benefits them. For example, one team is weak on RB this week (e.g., Charles is out) and the other team is weak at WR (e.g., Jeffrey) and the other team is strong at that position, but neither team wants to lose their guys long term. They help each other this week to the detriment of the rest of the league. If its anything like that scenario I think it should be vetoed.
Very strange scenario for sure.