Can anyone explain why people throw colours/water balloons on literal strangers?

I had my class 12 English exam today and while coming back home, some random bunch of teen boys threw water balloons on me. Mind you, I was in an auto, not even walking. My skirt, my admit card and the Question paper got fully drenched. I am not against having fun, but why do it to some random stranger? What if it was an old person or a sick person sitting there? What if they accidentally threw it at the auto driver? Is this normal in north india?