1600 sites, that sounds like fun!
It looks like the MX firewalls at each site possibly rebooted at roughly the same time, but it's hard to tell because there is no true "reboot" event for Meraki event logs. I'm seeing these same events at all sites:
Jul 6 11:03:13 Status Ethernet port carrier change device: port1, carrier: true
Jul 6 11:03:13 Status Ethernet port carrier change device: port0, carrier: true