Are you sure there was a big rise in dynamic languages at any particular point? I’m not so sure there really was (but I’m interested in documentary evidence if you know of any).
I always figured dynamic languages became more popular as computers got bigger and faster, so efficiency became less important in most scenarios. But that’s not the only trend -- plenty of people programmed in BASIC in the 80s, on tiny 8-bit computers! It was slow as hell, but P-code can be very memory efficient, so even on a tiny slow computer dynamic languages can make sense.
(I count BASIC as dynamic even though it lacks useful abstraction mechanisms because most dialects manage memory for you automatically.)
Edit to add: maybe Forth is a better example (and a much better language!) although I don’t think it was ever as popular as BASIC.