Two things.

One, through 1974 Nixon was well aware that there were more than enough votes in the House to impeach him but the case in the Senate looked much closer, with no way to be sure whether or not conviction and forced removal from office would occur. This all changed in August when, as a consequence of the ruling by the supreme court in US vs. Nixon, the full White House tapes were released. The tapes were effectively a "smoking gun" proving Nixon's guilt and his political support completely crumbled after their release. Having been only at the level of tenuous survival before, the collapse of political support meant that impeachment and conviction was a pretty nearly sure thing. Notably, several GOP reps who had voted against sending articles of impeachment for obstruction of justice to the full house indicated publicly that in light of the new evidence they would be voting for those articles in the full house vote.

Two, in the face of these realities a small contingent of GOP senators and representatives met with Nixon in the White House on August 7th and made it clear to him that they felt he did not have the political support to withstand impeachment. The following day Nixon resigned.