Likely came from your backup.
See the TCB News item which states:
Files likely in your backup that need update:
.xsession
.desktop
.desktop file is created during boot, but a backup will overwrite it.
Change it to be correct.
.xsession has a construct to use a default if specifiec WM in .desktop is missing.
DESKTOP="$(awk '/wm:/{print $2}' .desktop)"
"${DESKTOP:=flwm}" &