Filmmaking is a collaborative business. Between the editor, the producer, the production designer, the costume designer, the gaffer, the boom operator, and the camera operators, there are a lot of moving pieces. That means there are also a lot of personalities, and in a perfect world, those personalities would sync up, everyone would be on exactly the same page, nobody would be an a**hole, and everyone would sing Kumbaya.
Alas, the world isn't perfect, and film sets - just like every workplace - sometimes have a good deal of friction. Oftentimes, the most highly-publicized bouts of friction and tension come from the conflict between the director and one of their actors. In particularly contentious situations, directors and actors find themselves at each other's throats throughout an entire production, and sometimes even after the movie has wrapped, released, and hit home video.
The star and the director are two of the most prominent voices on any movie set, and everyone - especially creative types with a specific creative vision - can get headstrong.
And headstrong is fine, but when you don't budge for the sake of the collaborate project, then sparks start to fly.