Dell PowerEdge R440 Backplane Fire

Last week the R440 I have as part of my homelab unexpectedly cut out - I was only notified when our smoke detectors went off!  By the time I got to the server, the server was off with the PSU light flashing orange (was only using one of the dual PSUs at the time).  The power supply itself was connected to a UPS that hasn't reported any power spikes or other issues, and other equipment connected to the UPS has all been fine, suggesting this was not an issue with the input power.
The server wasn't under any abnormal load, and the disks connected to the backplane had not been added/changed/removed in close to a year.
Does anyone have any insight what could have caused this?  This seems quite a sudden and catastrophic failure that I haven't come across before, and it's got me a little nervous about the other Dell equipment I've been installing at clients!