3 of 4 added 10 characters in body

What is the founction of but in the sentence?

I don't know why but for some reason when I got on the train that day it was unusually full, something I don't recall ever happening in the past.

As for the above sentence, I have some questions:

  1. Is why a conjunction or adverb? My own understanding is that why is an adverb here which modifies the verb know, and but is the conjunction which connects two clauses. Is that true?

  2. As I read on Grammarly.com, when but is joining two independent clauses, we need to put a comma before it. Otherwise, leave the comma out. According to this rule, the two clauses are 'I don't know why' and 'for some reason when I got on the train that day it was unusually full, something I don't recall ever happening in the past.' But since there is no comma and the second clause doesn't express a complete thought, the second clause is a dependent clause or subordinate clause. Is that right?

  3. someone told me that the whole sentence is an object clause after why, but if the above clues are correct, then the sentence is not a object clause.