Her contract was up and she had a better paying job offer, with better billing. It was a professional decision to leave instead of renewing for a one year contract.
She was on Becker for six (six?) years.
Terry stayed for 4 years.
The show lasted 6 years.
No idea why she couldn't last the distance this time?
But a year later after Becker, she's seemed to have given up acting.
Wikipedia says Farrell's retirement was about concentrating on her family and mastering Yoga.