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Hard to say what might be in the works.
A CLI is one possibility, but negative actions might also be under consideration, such as CLD or closure.
Larger creditors develop and use their own internal scorecards and other indicators that effect their decision making on grant of credit limit (both for a potential increase or decrease), closing of accounts, etc., and some even develop their own internal scoring.
What is your current score, and what is your current payment history profile, especially for recent delinquencies?
While FICO does not use income as a scoring factor, creditors can and do use income as an internal decision making factor
Make sure that the email is legitimate first. I received a similar email back in December. I've been with WF for years and they'd never sent me an email like that before, so I called and asked. It was a phishing email, and a very good looking one at that. Just be careful.