Router sometimes hangs on reboot

[nug]

This problem has occurred intermittently and I'm wondering if it's possible to identify the cause. Basically the router appears to not complete the reboot after sending the signal via the web interface. Here's a copy of my logs from the most recent hang

Mar 30 15:47:13 syslog: pam_unix(sshd:session): session closed for user vpn
Mar 30 15:47:34 rc_service: httpd 653:notify_rc reboot
Mar 30 15:47:35 dnsmasq[647]: read /etc/hosts - 5 addresses
Mar 30 15:47:35 dnsmasq[647]: using nameserver xxx#53 for domain local
Mar 30 15:47:35 dnsmasq[647]: using nameserver xxx#53 for domain local
Mar 30 15:47:35 dnsmasq[647]: using nameserver xxx#53
Mar 30 15:47:35 dnsmasq[647]: using nameserver xxx#53
Mar 30 15:47:35 pppd[828]: Hangup (SIGHUP)
Mar 30 15:47:35 pppd[828]: Terminating on signal 15
Mar 30 15:47:35 pppd[828]: Connect time 535.0 minutes.
Mar 30 15:47:35 pppd[828]: Sent 93701588 bytes, received 110622548 bytes.
Mar 30 15:47:35 pppd[828]: Connection terminated.
Mar 30 15:47:35 pppd[828]: Sent PADT
Mar 30 15:47:35 openvpn[1243]: ac68u/xxx:63720 write UDPv4: Network is unreachable (code=101)
Mar 30 15:47:35 openvpn[1243]: ac68u/xxx:63720 write UDPv4: Network is unreachable (code=101)
Mar 30 15:47:36 dnsmasq[647]: read /etc/hosts - 5 addresses
Mar 30 15:47:36 dnsmasq[647]: using nameserver xxx#53 for domain local
Mar 30 15:47:36 dnsmasq[647]: using nameserver xxx#53 for domain local
Mar 30 15:47:36 dnsmasq[647]: using nameserver xxx#53
Mar 30 15:47:36 dnsmasq[647]: using nameserver xxx#53
Mar 30 15:47:36 pppd[828]: Exit.
Mar 30 15:47:36 kernel: Attempt to kill tasklet from interrupt
Mar 30 15:47:36 kernel: device eth0 left promiscuous mode
Mar 30 15:47:36 kernel: br0: port 1(vlan1) entering forwarding state
Mar 30 15:47:36 kernel: device eth0 entered promiscuous mode
Mar 30 15:47:36 kernel: br0: topology change detected, propagating
Mar 30 15:47:36 kernel: br0: port 1(vlan1) entering forwarding state
Mar 30 15:47:36 kernel: br0: port 1(vlan1) entering forwarding state
Mar 30 15:47:36 openvpn[1243]: ac68u/xxx:63720 write UDPv4: Network is unreachable (code=101)
Mar 30 15:47:36 openvpn[1243]: ac68u/xxx:63720 write UDPv4: Network is unreachable (code=101)
Mar 30 15:47:36 openvpn[1243]: event_wait : Interrupted system call (code=4)
Mar 30 15:47:36 openvpn[1243]: /usr/sbin/ip route del 10.8.0.0/24
Mar 30 15:47:36 openvpn[1243]: Closing TUN/TAP interface
Mar 30 15:47:36 openvpn[1243]: /usr/sbin/ip addr del dev tun21 local 10.8.0.1 peer 10.8.0.2
Mar 30 15:47:36 openvpn[1243]: PLUGIN_CLOSE: /mmc/lib/openvpn/plugins/openvpn-plugin-auth-pam.so
Mar 30 15:47:36 openvpn[1243]: SIGTERM[hard,] received, process exiting
Mar 30 15:47:38 iTunes: daemon is stopped
Mar 30 15:47:38 FTP Server: daemon is stopped
Mar 30 15:47:39 Samba Server: smb daemon is stopped
Mar 30 15:47:39 kernel: gro disabled
Mar 30 15:47:39 Timemachine: daemon is stopped
Mar 30 15:47:39 WEBDAV Server: daemon is stopped
Mar 30 15:47:41 kernel: usbcore: deregistering interface driver cdc_mbim
Mar 30 15:47:41 kernel: usbcore: deregistering interface driver cdc_ncm
Mar 30 15:47:41 kernel: usbcore: deregistering interface driver qmi_wwan
Mar 30 15:47:41 kernel: usbcore: deregistering interface driver cdc_wdm
Mar 30 15:47:41 kernel: usbcore: deregistering interface driver rndis_host
Mar 30 15:47:41 kernel: usbcore: deregistering interface driver cdc_ether
Mar 30 15:47:41 kernel: usbcore: deregistering interface driver asix
Mar 30 15:47:41 kernel: cdrom: Uniform CD-ROM driver unloaded
Mar 30 15:47:41 kernel: usbcore: deregistering interface driver usblp
Mar 30 15:47:41 kernel: xhci_hcd 0000:00:0c.0: remove, state 1
Mar 30 15:47:41 kernel: usb usb1: USB disconnect, address 1
Mar 30 15:47:41 kernel: xhci_hcd 0000:00:0c.0: USB bus 1 deregistered
Dec  1 11:00:13 syslogd started: BusyBox v1.20.2
Dec  1 11:00:13 kernel: klogd started: BusyBox v1.20.2 (2015-03-06 14:47:49 EST)
Dec  1 11:00:13 kernel: Linux version 2.6.36.4brcmarm (merlin@mint-dev) (gcc version 4.5.3 (Buildroot 2012.02) ) #1 SMP PREEMPT Fri Mar 6 14:56:01 EST 2015
Dec  1 11:00:13 kernel: CPU: ARMv7 Processor [413fc090] revision 0 (ARMv7), cr=10c53c7f
Mar 30 19:24:56 rc_service: ntp 989:notify_rc restart_upnp

As you can see this was the last process Mar 30 15:47:41 kernel: xhci_hcd 0000:00:0c.0: USB bus 1 deregistered and after that the router hung. I rebooted it via VPN whilst at work so I could only reset it once I got home. Any ideas?