They think you were a part of an organized group that was arbitrage betting, meaning that while betting on football, like you did, you calculate the profit and one of you bets on one team, while the other one bets against it. You do it in such way that if you get the favorable outcome you win money, despite your partner losing his bet, and if you get the unfavorable outcome, the win from your partner nullifies your loss. The more people in a group the more you can manipulate the odds and even make multi bets to increase profit.

Even if you're caught doing it, the remaining money should be returned to you and probably will, so stay calm and wait for more info from DD.