Sometimes the thing just goes wonky. Flippin the switch resets the flight director system and will do the trick 99% of the time.
Both switches must be switched off at the same time to re-cycle the system.
One thing that I just thought of is taxying out to the runway sometimes the F/D pitch command bar decides to stop indicating 8 degrees nose up, as it should in "TO" mode. The only way to clear the problem is switch both F/D switches OFF then return to ON again. This is for the 757/767 and is a known problem (documented procedure).
It's still quite rare though.
I used to love the smell of Jet-A in the morning...