Have you tried the ‘Wood’s Hole Observatory for non-repeatable events’?
Since the only thing in common to the servers is the UPS and probably the ‘Switch/Router’ they connect to. My first choice would be the UPS. Wake on LAN is easy, Die on LAN should leave traces in the event logs.
Are they sharing outlets (duplex wiring)?
Best guess is what ever tripped the first one will not be found, but the restore of the first tripped out the second and so forth. You didn’t specify the servers and some are much more sensitive to interruptions than others. Not necessarily a bad thing – data protection is the primary job.