Haha, I should have typed medical background?
As far as I know, unless it's coincidental, I doubt many decision writers have a medical background or training in the traditional sense. I'm sure they pick up or have familiarity with certain things (medical terminology, blue book listing criteria, etc.) once they've been working as a decision writer for awhile.
Heck, I know more than I want to about a lot of medical conditions and treatments just hanging around my 70+ year old mom, 90+ year old grandma, and all their friends. I also had never heard of some conditions and certain things like "FEV values" until I had been reading this forum for a year or so. I'm not comparing myself to a decision writer; I'm just saying one can pick up things, learn, be trained, etc.
Determining disability is a somewhat complex medical/legal evaluation. An ALJ makes a decision based on the available medical and non-medical evidence according to SSA laws and so forth. Decision writers write up most of the decision letters based on how the ALJ directs them (as far as I know).
That said, sometimes an ALJ makes an error and a claimant might get denied when the claimant should have been approved based on their evidence and the law. This is why there are different levels of appeals.