Is it a Gigabyte board?
Don't they have settings in the bios where the cpu has to be a certain temp before the fan kicks in? If you start it up from cold it might take a few mins for the temps to rise high enough for the fan to start.
Total guess that, but possible.
I was working with one on a C2D the other week, CPU got to 94'C and the PC just shut down, no harm to the chip either.