That's certainly one suspicious bet. If the guy would win that bet, that would most probably be investigated by the bookie's security department. I can't think of a reason why the guy made this bet and why he considered it safe? Was he drunk or high?
I disagree. There is nothing suspicious about it frankly speaking. The bookie's security department wouldn't bother investigating such bets since it clearly looks like a legit bet.
That gambler thought this was a sure bet just like many other gamblers and never expected a comeback in the 2nd half which is why he thought that it was easy money, but luck turned against him and screwed him over.
Many reasons can make his bet using a lot of money. It seemed that the man was too sure that his bet would give him a big win, so he bet a lot of money. But the truth was that he had lost too much. There are no easy bets, so if we want to bet, only use the money we can afford. The rest of us are just waiting for the game's results to come out and see if we win or lose. That's why betting on sports sometimes looks easy. We just choose the team we like, but the opposing team can put pressure on our team until our team finally loses.