It is probably DPMS. See if you DPMS enabled anywhere.
Also from the XF86Config-4 manpage:
Option "StandbyTime" "time"
sets the inactivity timeout for the standby phase of DPMS mode.
time is in minutes, and the value can be changed at run-time
with xset(1x). Default: 20 minutes. This is only suitable for
VESA DPMS compatible monitors, and may not be supported by all
video drivers. It is only enabled for screens that have the
"DPMS" option set (see the MONITOR section below).
Option "SuspendTime" "time"
sets the inactivity timeout for the suspend phase of DPMS mode.
time is in minutes, and the value can be changed at run-time
with xset(1x). Default: 30 minutes. This is only suitable for
VESA DPMS compatible monitors, and may not be supported by all
video drivers. It is only enabled for screens that have the
"DPMS" option set (see the MONITOR section below).
Option "OffTime" "time"
sets the inactivity timeout for the off phase of DPMS mode.
time is in minutes, and the value can be changed at run-time
with xset(1x). Default: 40 minutes. This is only suitable for
VESA DPMS compatible monitors, and may not be supported by all
video drivers. It is only enabled for screens that have the
"DPMS" option set (see the MONITOR section below).
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