Learn Swedish: Vad är det som ligger bakom nervositeten? - What is it that lies behind the nervousness?

Both are grammatically correct, but the cleft construction Vad är det som... (What is it that...) adds emphasis. It focuses the listener's attention on finding the specific, perhaps hidden, cause. It makes the question sound more probing and analytical than a simple Vad ligger bakom.

Can I drop the word som and just ask Vad är det ligger bakom nervositeten?

No, som is absolutely required here. In a cleft sentence, som acts as the relative pronoun (like "that" in English) connecting the two parts of the sentence. Because the thing you are asking about is the subject of the verb ligger, you cannot omit som.

Could I use är instead of ligger to say "is behind"?

You would be understood if you said Vad är det som är bakom, but ligger bakom (literally "lies behind") is a very strong, natural Swedish collocation. It is the standard idiomatic way to talk about the underlying cause of a situation, event, or feeling.

Why does the sentence use the definite form nervositeten (the nervousness) when English might just say "behind nervousness"?