An unreal scene recently took place in the middle of a McDonald's restaurant in the US. In this shocking video filmed by another customer and broadcast later by TMZ, an employee of the restaurant violently attacks a customer, who simply asked for a glass of water. The images of the altercation quickly went viral.On Instagram, the witness who shot the film explains the scene: I was eating my hamburger, the lady asked for a water cup and the supervisor shut down the soda machine because she wasn't letting her get a free soda.This led to a seriously violent fight. It got even worse when a second employee got involved, she appears to lose her temper after initially trying to drag the customer away to break up the fight.Maybe a secret ingredient literally drives its consumers crazy Getty ImagesDespite the beating, the customer continues to fight back, even after being dragged away by another server. A representative of McDonald's told TMZ that an investigation had been opened to determine the exact cause of the fight, before adding: We do not tolerate, in any way, the behaviour of the employees as seen in this video.Not an isolated incidentThis is by far not the first time that violence has occurred in a McDonald's restaurant in the US. Earlier this year in St. Petersburg, Florida, a customer attacked a female server because he could not find a straw at the condiment station. Gun violence is also prevalent at McDonald's restaurants, with over 70% of violent incidences in the fast-food chain involving firearms.So why does the McDonald's chain attract so much violence? Their extended hours could be one explanation since most violent incidences occur between 10 p.m. and 4 a.m. However other explanations remain a mystery for now...