The Rockies went 294-308 under Tracy.
"I was surprised," Bill Geivett, the team's director of major league operations, told The Associated Press. "You know, Jim and I go back a long time. We worked together for three different clubs.
"Basically, Tracy called me and told me his intentions and we talked about a lot of different things, but he had already made up his mind," said Geivett, who also worked with Tracy in Montreal and Los Angeles.
Energized by the young players and the challenge of fixing things, Tracy had said repeatedly the last several weeks that he wanted to fulfill the final year on his contract in 2013. But he changed his mind after meeting with Geivett for several hours on Friday and then mulling those discussions over the weekend.
Asked why Tracy resigned, Geivett said: "I don't think there was any one thing in particular that seemed to stand out, but you'd have to ask him that."
Tracy didn't return phone calls and texts from the AP.
Geivett said he wanted Tracy to return next season.
"I mean, that's how I started our meeting on Friday, that he was the manager of the club," Geivett said. "Like I said, it was surprising."
Geivett, however, didn't try to change Tracy's mind.
"His decision was made when he called me and I respected that," Geivett said.
Geivett said he had no timetable for hiring a new manager: "All the focus has been on Jim Tracy the last few days here and I just got the call today, so we'll start to formulate a plan."
The Rockies will be the fourth team to change managers this year. Boston fired Bobby Valentine, Cleveland dismissed Manny Acta and Houston let go Brad Mills.
Things changed for Tracy on Aug. 1 when Geivett, the assistant general manager, was given an office in the clubhouse and began focusing on roster management, particularly as it related to the pitchers, and evaluating the coaching staff and the rest of the players.
Tracy's responsibilities were narrowed to game management and meeting with the media.
Geivett said that structure will remain in place next season.
In addition to altering their front office, with general manager Dan O'Dowd focusing his attention on the minor leagues and player development, the Rockies last summer adopted a radical four-man rotation and a 75-pitch limit with several designated piggyback relievers, an experiment that lasted two months.
Geivett said the Rockies will return to a traditional five-man rotation next season.
with pitch limits determined on a case-by-case basis, "although I don't think we'll ever go back to the days of 120 pitches."