S6720 restarts suddenly

The possible cause is that the power supply hardware is faulty.
You can check the restart information and whether hardware alarms are generated.
display reboot-info
display alarm urgent

< S6720>display reboot-info

Slot ID Times Reboot Type Reboot Time(DST)
==================================================
0 1 MANUAL 2019/09/24 05:43:32
0 2 POWER 2019/09/24 05:37:58
0 3 POWER 2019/09/24 05:19:35
0 4 POWER 2019/09/24 04:57:05
0 5 POWER 2019/09/24 04:36:56
0 6 POWER 2018/04/26 11:11:57
0 7 POWER 2018/04/12 18:53:38
0 8 POWER 2018/03/30 15:17:09
0 9 POWER 2018/03/17 18:46:44
0 10 POWER 2018/03/16 13:13:29
0 11 POWER 2017/12/01 22:05:37
==================================================
Total 11

< S6720>display alarm urgent
Alarm Slot Date Time(DST) Location
---------------------------------------------------------------------------
Power normal 0 2019/09/24 05:51:11 Slot0
PWR Input Vol Low Resume0 2019/09/24 05:51:09 Slot 0PWR1
Powerabnormal 0 2019/09/24 05:47:25 Slot0
PWR Input Vol Low 0 2019/09/24 05:47:24 Slot 0PWR1
Powerabnormal 0 2019/09/24 05:38:01 Slot0
PWR Input Vol Low 0 2019/09/24 05:38:01 Slot 0PWR2

PWR Input Vol Low: A power module is in an input power outage or undervoltage condition.
PWR Input Vol Low Resume: A power module recovers from an input power outage or undervoltage condition.
It shows power input voltage low and it recovers after a few seconds. So I suggest you check the power supply and swap power module first.