I've seen this happen when things like uninitialized pointer errors, or fatal errors in constructors for the static classes. Something really early in the boot process that kills everything before driverstation stuff can happen. I usually catch it with NetConsole, which will give you all the output from the console, including vxWorks boot stuff. Here's the screensteps for it and RioLog:
http://wpilib.screenstepslive.com/s/...console-output
I haven't used RioLog before, but it sounds like it should give you the same level of output. You DO need enable Console Output in the Startup Settings on the home page of the roboRIO, though. If you do get the console output during the boot, there's going to be some sort of fatal error near the end that should hopefully make things a little more obvious.