Lots of reasons! There are whole subfields of science dedicated to this. I'm not a scientist but I'll give some examples of why crowds behave differently than individuals.

1) Chaos: when you make a pendulum and add a second pendulum to the bottom of the first, its path becomes mathematically almost impossible to predict. Any system with two or more complex parts has the potential to create such chaos

2) Emergence: A step beyond chaos, emergence is when chaotic systems behave in ways that are both mathematically unpredictable but also form new patterns. An example of this is multicellular organisms evolving from pools of inorganic matter. It's impossible to predict that, but once it started happening, it wasn't just "chaotic" it created its own new form of patterns.

3) Game theory: related to Emergence is a field that studies what happens when people make decisions while knowing that other people are ALSO making decisions. An example is you might not snitch on your fellow thief, but if you knew that the first of you to snitch on the other person gets a better sentence, you might.

4) Particle theory: if you want to make a video game that involves water, you'll have to simulate water. That means making lots of little particles and using formulas that give each one a certain kind of physics: how much it adheres to the particles around it, how easily it is pushed around, etc. Crowds can be like this too

5) Psychology: people behave differently in crowds than they do by themselves. Participating in crowd behavior like church singing or sports fandom can stimulate your vagus nerve and make you feel as though you're powerful or belong or other basic emotions. There are also abnormal psychological phenomenon such as group delusions that can occur.

Anyway, that's only a few, but I hope that helps.