...WHERE did they come from?
It's like at that time where did all their technique come from?...
I know one factor was immersion schools like GIT (later MIT). A lot of '80s shredders either went there or were taught be guys who went there. Before GIT, there were legit music colleges like Berklee, but I believe they centered on jazz and classical more than rock.
I was a child of the '70s, and was taught by several good teachers in my town, but they were primarily jazz or fusion players. (Rock stuff was easy for them.) I was at the top of my game in the early '80s and played pretty well, but then guys started leaving town to go to GIT and they were coming back able to run circles around me.