Pension fund Drawdown & tax code

I am in full time employment and will be retiring next year at 66 . I have a drawdown pension and make a small withdrawal once per year. Last year I withdrew £2000 which was taxed but my tax code changed from 1270L to something like 355X . I could not contact HMRC via phone so wrote a letter . It was eventually sorted out. Last week I withdrew £6000 which was taxed but same again this changed my tax code from 1271L to minus £1500 . Can someone please tell me why this happens.

Just an update to this, my tax code changed from 1271L to 155X . After I contacted HMRC it shows on the HMRC App my tax as been put back to 1271L but my employer has not received the new code and are having to use the 155X code. I’m paying double amount of tax I should be paying.

Hi JOHN LLOYD,
The amounts may be incorrectly converting to an estimated annual amount which would cause your allowance or tax rates to be altered.
If you have signed up to the Personal Tax Account you can update the estimated income and this would amend the code accordingly.
Thank you, 
 

Hi JOHN LLOYD,
If you  please contact our helpline we can reissue the tax code:
Income Tax: general enquiries
Thank you. 
 

The contact numbers for HMRC you linked are a utter waste of time and money . An automated voice that asks for security information then at the end of the call says goodbye before you get to speak to a human. I end up writing a letter to PAYE & Self Assessment, HM Revenue & Customs , BX9 1AS , who seem to sort things out pretty quickly.