'As' is being used instead of 'because' in this sentence.

This is a quote from a lovely old dictionary called Success With Words. "AS" is sometimes used to mean "since" or "because". Some usage experts dislike this, feeling that it is a weak method of referring to a reason and also that it is apt to be mistaken for "when". It would be better to say, "He was completely out of breath because he had run all the way to the station" Hope this helps.

As = since = because. "As" and "since" are more formal than "because". We usually put a comma before since after the main clause.