FYI, this is a network issue, as seen in this excerpt:
[ 621.135763] tegradc 15200000.nvdisplay: dc_poll_register 0x41: timeout
[ 621.135932] tegradc 15200000.nvdisplay: dc timeout waiting for cursor act_req
[ 640.547619] NMI watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [swapper/0:0]
[ 640.548380] Kernel panic - not syncing: softlockup: hung tasks
[ 640.548540] CPU: 0 PID: 0 Comm: swapper/0 Tainted: G L 4.9.140-tegra #1
[ 640.548680] Hardware name: Jetson-AGX (DT)
[ 640.548775] Call trace:
[ 640.548848] [<ffffff800808bdb8>] dump_backtrace+0x0/0x198
[ 640.548959] [<ffffff800808c37c>] show_stack+0x24/0x30
[ 640.549068] [<ffffff800845c7a0>] dump_stack+0x98/0xc0
[ 640.549179] [<ffffff80081c1438>] panic+0x11c/0x298
[ 640.549285] [<ffffff8008181760>] watchdog_unpark_threads+0x0/0x98
[ 640.549416] [<ffffff80081399e0>] __hrtimer_run_queues+0xd8/0x360
[ 640.549532] [<ffffff800813a330>] hrtimer_interrupt+0xa8/0x1e0
[ 640.549653] [<ffffff8008bfea80>] arch_timer_handler_phys+0x38/0x58
[ 640.549776] [<ffffff8008126f10>] handle_percpu_devid_irq+0x90/0x2b0
[ 640.549897] [<ffffff80081214f4>] generic_handle_irq+0x34/0x50
[ 640.550253] [<ffffff8008121bd8>] __handle_domain_irq+0x68/0xc0
[ 640.550732] [<ffffff8008080d44>] gic_handle_irq+0x5c/0xb0
[ 640.551891] [<ffffff8008082c28>] el1_irq+0xe8/0x194
[ 640.556790] [<ffffff8008decfa8>] netlink_broadcast_filtered+0x60/0x440
[ 640.563177] [<ffffff8008ded3d8>] netlink_broadcast+0x50/0x68
[ 640.569062] [<ffffff8008def778>] nlmsg_notify+0x68/0x120
[ 640.574372] [<ffffff8008dbf86c>] rtnl_notify+0x5c/0x70
[ 640.579191] [<ffffff8008e8fb38>] ndisc_router_discovery+0x838/0x9d0
[ 640.585487] [<ffffff8008e901a4>] ndisc_rcv+0xec/0x668
[ 640.590740] [<ffffff8008e989a4>] icmpv6_rcv+0x374/0x568
[ 640.596160] [<ffffff8008e75e94>] ip6_input_finish+0xe4/0x4f0
[ 640.601677] [<ffffff8008e762d8>] ip6_input+0x38/0xb8
[ 640.606922] [<ffffff8008e76930>] ip6_mc_input+0xc8/0xf0
[ 640.612171] [<ffffff8008e75d24>] ip6_rcv_finish+0x64/0xf0
[ 640.617252] [<ffffff8008e76698>] ipv6_rcv+0x340/0x510
[ 640.622676] [<ffffff8008da8810>] __netif_receive_skb_core+0x3b8/0xad8
[ 640.629150] [<ffffff8008dabc00>] __netif_receive_skb+0x28/0x78
[ 640.635185] [<ffffff8008dabc7c>] netif_receive_skb_internal+0x2c/0xb0
[ 640.641225] [<ffffff8008dac8a4>] napi_gro_receive+0x15c/0x188
[ 640.647178] [<ffffff800894dd90>] eqos_napi_poll_rx+0x358/0x430
[ 640.652950] [<ffffff8008daded4>] net_rx_action+0xf4/0x358
Perhaps it is related to IPv6 (which is not tested nearly as well as IPv4), but I could not tell you how to track the specifics.
For reference, the similar but “not same” thread is:
https://forums.developer.nvidia.com/t/jetson-agx-xavier-self-rebooting/148000/22
In the other thread he is having network related reboots. It is difficult to say what the specific issue is, but perhaps it is partially data driven. Someone else may know how to look closer that network issue, and this in turn would probably lead to more information about either overheating or reboot.