To really understand why Maniac is upset with himself for crossing the finish line backwards, you really have to understand Mars Bar. Mars Bar is the leader of his neighborhood gang and has been bullying Maniac for some time. He is a big talker, always bragging about himself and putting others down. Mars Bar is notorious in the neighborhood for picking on little kids and adults alike.
On the day the race takes place, Mars Bar is talking trash to Maniac and showing off his new athletic shoes. He challenges Maniac to a race. The whole neighborhood gets into the race. Traffic is blocked on both ends of the street. The audience is composed of mothers, fathers, little kids, and high school students--everyone's there!
As the race begins, Maniac runs slowly. Even though he wants to win, he knows there will be consequences if he does. But as he nears the finish line and sees Mars Bar winning, Maniac's instincts take over and he speeds up, passing Mars Bar and crossing the finish line running backwards. The crowd goes wild; everyone's talking about how Maniac not only beat the neighborhood bully, but beat him running backwards!
Naturally, Mars Bar gets angry. He gets mean. He starts making excuses and blaming others for his loss and shoving people around. Maniac realizes that winning the race was bad enough, but having humiliated Mars Bar in front of the whole neighborhood is going to make things a lot worse for himself.