Now that I'm home and can look at the actual logs, it appears that the actual order of events was battery - line - battery - power failure - restart. As such, the missing event that should have happened is the UPS telling the fileserver that it was time to shut down before it ran out of juice.

Reading over postings on the Infrant support BBS, it seems that my UPS isn't officially supported, but it could well be emulating an APC. The next step is going to be installing the Windows-specific software and seeing if I can reconfigure the UPS to send the shutdown message sooner...