This is an interesting, but old, audio news piece about how fear spreads among crowds and leads to stampedes, etc. "Human beings are hardwired to respond to fear of others." "Fearful body language primes humans to flee."
Interestingly, another article suggested that bad decisions may also be contagious.
As a side note, being in a crowded nightclub when a fire breaks out (story from the audio link), or any similar mob type situation is the reason why I experience anxiety in crowded spaces.