If I have to categorize them I would put 난데없이/뜬금없이, 갑자기/갑작스러움 together and 어느새, 예상치, and 불쑥 separately. 어느새 is probably a compound of 언제+사이. So used more in the situations where you would express 'slowly approaching' or 'without realizing it' 예상치 is short word for 예상+하지(하다) most fitting word to unexpected. 예상 means expectation or forecast. It's used as like this : 전혀 예상치 못 했던 일/ 예상치 않은 일. NOT always follows up, and you should not finish the sentence with it. Treat it like 예상하지. (Although you can finish sentence with 예상하지 in dialogue :"다들 그럴거라고 예상하지") 불쑥 is more an action describing word. Popping out of, or protruding. Can be use with other words like: 갑자기 불쑥/ 난데없이 불쑥...and such 난데없이/뜬금없이 they can be used either way. They have more meaning of 'out of nowhere' I assume 난데없이 is a short term for 나온 곳 없이/낳은 곳 없이. For 뜬금없이 saw an article that it's related to market price somehow. 갑자기/갑작스러움. Basically the same thing. They mean suddenly/sudden. If you're interested in random Korean word origins or grammars and such, follow National Institute of Korean Language twitter @urimal365. They answer all sorts of question and it's fun to just read.