Possibly because some graphical components has been updated - forcing a window manager restart.
The recommendation is to switch to a TTY and do the system sync there as doing this will ensure the sync process is not interrupted by a window manager reset.
→ [root tip] [How To] Update Manjaro the smart way
→ [HowTo] Updating Manjaro as safely as possible
Also ensure that you are NOT updating custom scripts in the same transaction - always split into
sudo pacman -Syu
Or
pamac upgrade --no-aur
And rebuild custom packages when the above succeeds
pamac upgrade --aur
Custom build scripts is intirely unsupported.
→ [Need-To-Know] About Manjaro and AUR
Disable AUR - it is know that Pamac may cause excessive resource usage when AUR is enabled in the GUI.
Disabling AUR will not change the ability to build custom packages from the command line.