What is it with people throwing s**t from/at cars?

I've never personally seen this happen outside of Houston. Specifically this seems to be a recurring theme on 290. Today some piece of s**t pulled out of the Walmart parking lot, cutting me off on the feeder road. I tried to go around them so they intentionally sped up to block me. I pulled back behind them in the middle lane and they could tell I was irritated because they started looking back at me in their side mirror laughing and gesturing for me to come at them. I just held up my thumb and index finger to indicate something tiny and then got into the far left lane. Just before I got on the entrance ramp they gunned it and threw something at my car and hit it pretty hard.

Why are people here such bullying pieces of s**t? How do you deal with situations like this? I have a dashcam, but since he was alongside me and not in front of me there's nothing to see.