Learn Finnish: Uusi sarja osoittautui yllättävän koukuttavaksi. - The new series turned out to be surprisingly addictive.

This is due to the verb osoittautua (to turn out to be). This verb has a strict rule (a rection): the adjective or noun describing the outcome must be in the translative case (-ksi). It emphasizes the result or the state that was eventually revealed.

I know yllättää means "to surprise", but why is it in the genitive form yllättävän here?

Here, it's acting as an adverb meaning "surprisingly." In Finnish, you can take an adjective or a participle—like the present active participle yllättävä (surprising)—and put it in the genitive case (-n) to make it an adverb that modifies another adjective. It tells us how addictive the series was.

Does the adverb yllättävän need to match the case of koukuttavaksi?

No, it doesn't. Because yllättävän is acting as a modifying adverb ("surprisingly") rather than an adjective directly describing the series, it stays in its fixed adverbial form. Only the main adjective koukuttavaksi takes the translative case required by the verb osoittautui.

How is the word koukuttavaksi formed?

The root is the noun koukku (a hook). From that, you get the verb koukuttaa (to hook someone / to make addicted). By adding the -va participle ending, it becomes the adjective koukuttava (addictive / hooking). Finally, the -ksi ending is added to form the translative case.