Just having read your post properly it sounds very similar to what happened to me. On Route 7 the of road section down to Rookhope is so steep and rubbly, my wrist pain got so intense I was unable to brake.
I've stayed away from that section since but getting back on thin bar, drops road bike from my thicker bar hybrid also caused a similar very high intensity of pain, no rough surface or brake usage even needed.
Interestingly I'm fine on the road bike now so proper usage toughens/flexes you up in old age.