Perhaps it was because he was now so busy, what with Quidditch practice three evenings a week on top of all his homework, but Harry could hardly believe it when he realized that he'd already been at Hogwarts two months.
I have 2 questions about the sentence:
I don't quite understand the usage of 'what', because I feel it's fine to write: Perhaps it was because he was now so busy with Quidditch practice three evenings a week on top of all his homework, ...
I doubt the usage of 'but', I think 'so' seems make more sense to me. E.g.
Perhaps it was because he was now so busy, what with Quidditch practice three evenings a week on top of all his homework, so Harry could hardly believe it when he realized that he'd already been at Hogwarts two months.
P.S. The sentence is from Harry Potter.