To resolve this, go through the re-authorisation process.
There are multiple reasons the prompt could keep appearing:
The browser's cookies and history is being cleared or deleted.
Upon a successful authorisation, a cookie is placed on the machine, within the used browser. However if the browsers cookies and cache, or history is being cleared, this will then reset the authorisation with HMRC's system.
If your IT has a policy set where your browser cookies are deleted after a certain period, you would need to ensure the HMRC cookie is whitelisted to stop this happening.
Changing the web browser used from the last session will also trigger the re-authorisation process.
HMRC's system recognises and logs which browser, location and PC or laptop you connect from each session. If any of these change, it will trigger the re-authorisation process.
If HMRC were having issues on their side and their system fell over or went offline, they may require re-authorisation once it comes back up.