Translation:What is the reason for their failure?
It is much worse than you think.
In Polish, every verb structure requires an Object, or Direct Object and Indirect Object in certain grammatical cases. Sometimes there are 2 cases possible, each giving the phrase a different meaning (even if there are the same nouns involved). Sometimes the verb structure is just a verb.
In this case, the verb structure is composed of interrogative pronoun jaki + verb jest. The structure jaka jest requires:
\1. either an Object in Nominative (usually noun or noun phrase)
- Direct Object in Nominative (here: przyczyna)
- and Indirect Object. If there is NO modifier, the Indirect Object has to be in Genitive case (in this case, there is no modifier). The modifiers can be prepositions, eg. "na" changes the case to Accusative, "w" changes the case to Locative, "pod" changes the case to Instrumental , etc.
In this sentence the Indirect Object is ich porażka. Luckily, that the pronoun ich (= their or theirs) does not have any declension, it is just ich in every grammatical case.
And here is declension of porażka (= failure, defeat, beating)
- Nominative (mianownik) (sing.) porażka - (pl.) porażki
- Genitive (dopełniacz) (sing.) porażki - (pl.) porażek
- Dative (celownik) (sing.) porażce - (pl.) porażkom
- Accusative (biernik) (sing.) porażkę - (pl.) porażki
- Instrumental (narzędnik) (sing.) porażką - (pl.) porażkami
- Locative (miejscownik) (sing.) porażce - (pl.) porażkach
- Vocative (wołacz) (sing.) porażko - (pl.) porażki
As you see, the Genitive singular is the same as Nominative plural: porażki. That is the case of many nouns in Polish language. So if you see someting that looks like a Nominative plural, you'd better know what is the case which is required by the verb, because that word might be a Genitive singular instead.
P.S. The verb jest alone requires either Nominative or Instrumental, depending on type of phrase.