Maintenance Center (don't know if that is the correct name in English) suddenly alarmed me that AVAST FREE and Windows Defender were offline without me doing anything.
Your question is a fair question. It should not normally do that.
While it's possible that you may see a message from the Action Center
during an Avast major update, I've never seen what you saw as a typical occurrence "out of the blue" (i.e., at any time other than during an actual Avast software update).
Normally you do NOT have to do anything yourself; Windows will automatically switch off it's anti-malware software when it detects Avast on the job.
-Noel