Or if you don't like silly internet stuff, here is the normal version of the song.
Why do you think it should be in the accusative?
It's the subject of the sentence.
Remember that you can usually ask after the subject with "who or what [verb]s?" -- and here you are literally asking "What is happening?", i.e. using "what?" to ask after the subject.
The subject of a sentence is in the nominative case, without an ending.