This is the 11007th most frequent Korean word.
급등했던
"급등했던" means "that surged" or "that rapidly increased."
Here, '급등했던' is used as the past attributive form of 급등하다 (to surge) to describe the stock price.
지난주에 주가가 급등했던 이유는 무엇인가요?
What was the reason that the stock price surged last week?
In this sentence, '급등했던' describes the popularity that has peaked in the past.
그의 급등했던 인기는 지금은 안정되었습니다.
His popularity, which had surged, has now stabilized.
Here, '급등했던' is used to denote the rising of prices in the past, attributing the event to subsequent difficulties.