Reportedly, it was Shannen Doherty and co-star Alyssa Milano's relationship that was the issue. "There were times when I'd come in and say, 'Good morning, Shannen,' and she didn't say anything to me," Milano admitted (via E! News). "And there were times when she'd come in and say, 'Good morning, Alyssa,' and I wouldn't say anything to her." Though the two originally got on like a house on fire when Charmed started filming, things seemed to decline pretty quickly. 

Expanding on their issues, Milano told Entertainment Weekly, "I think it's hard when you put ... [long pause] ... two very different people together. I'm very laid-back and passive. I have my Buddha. I come in here and meditate. [Shannen's] got a lot of energy, she's very headstrong, she wants to get the job done." 

Ultimately, the clashing of their personalities seems to have been the reason Doherty chose to leave.