Because people do it as a joke when in reality even a 10mph crash could end your life on a motorcycle. Drivers get hostile for all sorts of reasons towards motorcycles. Jealousy, having a bad day, or even just general distaste for motorcycle riders.
Usually you can see it coming and avoid the situations but it is always a serious situation for the rider. I ride both evasive and agressive at same time.
I hold my line but always scan for the way out in case things go south. It is a completly different view of the road when your on a motorcycle.