There are a couple of obvious explanations for why people buy more stuff on Black Friday and Cyber Monday:
1. Retailers offer better prices enticing people who have been thinking about buying to finally pull the trigger.
2. People are helpless victims of mass advertising and are deluded into thinking that they are acting along with their many friends.
I have one more possible reason to add to this list: people look for excuses to follow their desires rather than use restraint. We see a similar effect with people who try to eat healthily. Even people with good salaries who can afford to eat anything they want will get overly excited about free donuts or pizza. They can afford to buy junk food, but they usually show restraint and eat healthy food. The free junk food is an excuse to indulge.
So, for people who have a desire to shop, Black Friday and Cyber Monday are a great excuse for going off their "diets".