I think the event is a bad example to tell why people are littering. When lots of people come together, the streets get dirty. Even if there are enough trash cans. Once they are full and people start putting cans and paper bags underneath, there will be someone who kicks it a few feet away, maybe accidently maybe it was the wind or animals at night and some may think, the area will be cleaned up anyway.
However, I can tell from the city and country I live that the level of garbage thrown at streets and sidewalks cannot probably tracked to one particular reason. It's a combination of a number of different reasons, I guess.