Over a four minute, 36 second period beginning around 10:20 PM EST, seven servers, all in one rack, all on the same UPS suffered unexpected shutdowns. All servers are time synchronized and examining the Event Viewer only two shutdown at exactly the same time. Two other racks of servers, each on their own UPS units did not shutdown. The UPS on the rack that shutdown has two, two year old batteries in it and was running at 30% of capacity. While I did not have any UPS management software running on this UPS, I did have it on the other racks' UPS units. Their logs showed no unusual electrial events, e.g., brownouts or spikes.
This had never before occurred and has not repeated itself. Does anyone have any idea of what could cause this?