I am not a native English speaker, so I try to explain the case as clear as possible.
Some people use the very good and expensive equiptments, expecially with heavy horses.
So the upkeep cost should be very high.
But what they are doing is, quit the game before the round end, so they do not have to pay any upkeep cost, but still earn the regular income, and uses the most expensive equiptments!
Of course wether they quit the game is depend on the situation, if their team is going to win, they will stay and enjoy X2 X3 etc...
So, I hope this bug be solved soon...