I just had the same issue today with the latest security update. The update was stuck for a long time. I had to force shut-down and my Mac mini behaved weirdly upon restarting.
Apps wouldn't launch, stuck mouse pointer etc., overall system speed down to a crawl. Had to restart and re-download the update and it worked after two more restarts.
Never had an issue with my Mac before (ran 10.14.6 for months).
It finally worked, but I would advise not to update for now and wait a few days.
This latest update caused the problem for me (just released on Nov 12, 2020):
Security Update 2020-006 High Sierra, Security Update 2020-006 Mojave
PS: Apple already pulled an update for Mojave some weeks ago after similar issues were reported:
According to reports from multiple users, those who installed Safari 14 or the security update have been experiencing varying issues.
www.imore.com