twest If fm runs wp-cli it could be a specific site has malware. I had an issue when testing where if an infected site had an infected wp-settings.php and you select the site in the control panel, cpu usage would be very high.
Selecting the site would trigger wp-cli processes to load plugin lists etc. The malicious code in wp-settings crashed wp-cli which would immediately launch the command again.... loop.... even after you closed the webpage.
Edit: Here is the case I was thinking of https://talk.plesk.com/threads/wp-toolkit-100-cpu-when-running-core-info-on-a-broken-wp-instance.349105/
If part of the backup process is wp-cli export for the db, some malware could cause high resource usage I imagine.