The aircraft I work on has similar authorities to yours and also has a restricted g-onset rate for positive g manoeuvres. If my memory is correct, the requirement to constrain the g-onset rate derived from some F16 accidents in the '80s, although I don't have any documentation supporting this and can't identify the accidents in question.
I believe that were no requirements to limit the g-onset rates in negative manoeuvres.
My understanding, and I am not an aeromedical expert, is that achieving high g very rapidly and then holding it for some seconds can lead to loss of conciousness without the usual warning symptoms (i.e. tunnel vision, grey out etc). This understanding seems to be supported by the report at the following link, particularly Figure 1, although that graph may apply to pilots without g-protection.
Link
I have never felt in possession of all the facts on this topic. I will be very interested to see what this thread can unearth.