You don't have one of those KickFlash things to prevent OS3.9 rebooting?
Other things might be:
* A problem with the RDB
* 2x devices with the same boot priority
* 2x devices with the same SCSI ID
* Master/Slave jumper
* BlizKick?
* There's a magnetic field buggering with your machine
* Overheating
* Tarnished contacts somewhere
* SIMM trouble
If it doesn't happen again just put it down to US military satellites trying to destroy all the Amigas left.
