I think there was just a lot of rivalry and they couldn't get on and that's why Shannen Doherty left because she said it was either her or Alyssa that had to go.
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He left because he didn't get along with Shannen Doherty (Prue).
Shannen Doherty
Shannen Doherty played Prue Halliwell in Charmed.
Shannen Doherty and Alyssa Milano hated each other and Doherty said if they didn't sack Milano, (Pheobe), she would quit.
Alyssa Milano and Shannen Doherty had disagreements but there were never real fights.