My understanding is that a group of Republican Congressmen went to Nixon and told him that he would be impeached and convicted. Rather than go through that process, Nixon opted to resign.

Because two reporters from the Washington Post named Bob Woodward, and Carl Bernstein wrote an investigative report that revealed that Richard Nixon was involved in the cover up during the Watergate Scandall. The Democratic Headquarters at that time was located at the Watergate Hotel. Nixon authorized a break in and was involved in the cover up during covert operations to infiltrate the Democratic national headquarters. Nixon ultimately won re-election but when the scandal was revealed and when Nixon was named as a key player in the covert op, he resigned because he was going to be impeached.

A couple people in a group that supported Nixon broke into the Democratic Headquarters located in the Watergate hotel. Nixon had no idea that this went on, and was in no way connected to the crime. However, when he was told about what had occurred, instead of letting everything run its course, he asked what it would take to keep this covered up. Because he was a bit paranoid, he had a recording system in his office, which caught this conversation. Using this evidence, there was reasonable evidence to convict him on this. Instead of undergoing the long, agonizing process of impeachment (which was imminant) he resigned.

A strategic move prior to impeachment?

Wasn't it because he was a crook?

Because he was told that enough votes existed to convict and remove him. The reason that a vote against him should take pace was the Watergate scandal. "In August, the previously unknown tape from June 23, 1972, was released. Recorded only a few days after the break-in, it documented Nixon and Haldeman formulating a plan to block investigations by having the CIA falsely claim to the FBI that national security was involved. The tape was referred to as a "smoking gun." With few exceptions, Nixon's remaining supporters deserted him. The ten congressmen who had voted against all three articles of impeachment in the committee announced that they would all support impeachment when the vote was taken in the full House. It was almost certain that Nixon would be impeached by the House and removed from office by the Senate. Throughout this time, Nixon still denied any involvement in the ordeal. After being told by key Republican Senators that enough votes existed to convict and remove him, Nixon decided to resign. In a nationally televised address on the evening of August 8, 1974, he announced he would resign, effective at noon Eastern Time on Friday, August 9, 1974. Though Nixon's resignation obviated the pending impeachment, criminal prosecution was still a possibility. He was immediately succeeded by Gerald Ford, who on September 8, 1974, issued a pardon for Nixon, immunizing him from prosecution for any crimes he may have committed as President. Nixon proclaimed his innocence until his death, although his acceptance of the pardon was construed by many as an admission of guilt. He did state in his official response to the pardon that he "was wrong in not acting more decisively and more forthrightly in dealing with Watergate, particularly when it reached the stage of judicial proceedings and grew from a political scandal into a national tragedy."" Source and further information: http://en.wikipedia.org/wiki/Watergate_scandal#Articles_of_impeachment.2C_resignation.2C_and_convictions

Because he got caught in Water Gate and would have been impeached anyways. Better to step down in shame then wait to be thrown out in shame.