Your interpretation is correct. The relative pronoun "what" refers to the reason for being here. The verb "to owe" works well with both a direct and an indirect object. In this sense it acts like the verb "to give". For example, "Jim gives a dollar to Bill" "Jim owes a dollar to Bill" So you can ask "We owe the pleasure to what?" though it sounds more natural to put the interrogative at the beginning of the sentence: "To what do we owe the pleasure?"