There was a Shadow on Númenor, and it began in the days of Tar-Ciryatan, the Twelfth King, and his son, Tar-Atanamir, the Thirteenth. The Line of Elros, from Unfinished Tales, summarises it best:
In his time the Shadow fell upon Númenor; and the King, and those that followed his lore, spoke openly against the ban of Valar, and their hearts were turned against the Valar and the Eldar; but wisdom they still kept, and they feared the Lords of the West, and did not defy them. Atanamir is called also the Unwilling, for he was the first of the Kings to refuse to lay down his life, or to renounce the sceptre; and he lived until death took him perforce in dotage.
The ban of the Valar was, of course, the prohibition against sailing "so far westward that
the coasts of Númenor could no longer be seen" (Akallabeth), and was compounded by the Númenorean's desire for immortality (also from the Akallabeth):
Why should we not envy the Valar, or even the least of the Deathless? For of us is required a blind trust, and a hope without assurance, knowing not what lies before us in a
little while. And yet we also love the Earth and would not lose it.
This ultimately erupted into open rebellion against the Valar, even before the time of Ar-Pharazôn; the Line of Elros (again) notes of the Twentieth King:
...He was the first King to take the sceptre with a title in the Adûnaic tongue; though out of fear (as aforesaid) a name in Quenya was inscribed in the Scrolls. But these titles were held by the Faithful to be blasphemous, for they signified "Lord of the West", by which
title they had been wont to name one of the great Valar only, Manwë in especial.
The origin of this Shadow must obviously lie in Melkor's original corruption of Men during the First Age, which the Númenoreans were evidently not fully healed of, and the fear of death and desire for immortality left them ripe for a second fall on Sauron's arrival.