Definition of "Herd Mentality" via Wikipedia:
"Herd mentality, or mob mentality, describes how people are influenced by their peers to adopt certain behaviors, follow trends, and/or purchase items. Examples of the herd mentality include stock market trends, superstition and home décor... Driven by emotional reactions such as greed and fear, investors can be seen to join in frantic purchasing and sales of stocks, creating bubbles and crashes."