No explanation from her was ever given, but apparently she had a huge opioid addiction.

Why did this beloved A/A- list mostly television actress walk away from a very popular network show that has several other spinoffs? No explanation from her was ever given, but apparently she had a huge opioid addiction. Being on a set every day for long stretches just gave her too much opportunity to keep taking them. She said she needed a change and several months of rehab and then outpatient treatment.