When the sun rose on August 25, the upstart Mariners were sitting pretty. Thanks to a streak in which they won 13 out of 17, they were 73–53, one game behind the Angels in the AL West, and three ahead of the Yankees in the wild card race. According to Baseball Prospectus’ Playoff Odds report — which uses a team’s run-scoring and run-preventing proclivities in a Monte Carlo-simulation that plays out the rest of the season one million times — the M’s held a 29% shot at winning their division, and a 30% shot at the AL Wild Card.

Less than three weeks later, the Mariners’ ship has all but sunk. A 1–13 skid helped knock them 8.5 games back in the division, and 5.5 back in the wild card, plunging their Playoff Odds down below 2%. In terms of raw wins and losses, they’ve set a dubious record — no team so far above .500 so late in the season has ever collapsed so quickly. What went wrong?

In retrospect, it’s surprising the Mariners contended at all this year. The team that made the playoffs four times between 1995 and 2000, and averaged 98 wins a year between 2000 and 2003, has fallen on hard times, with four straight losing seasons and a slew of questionable free-agent signings by general manager Bill Bavasi. Back in the spring, Baseball Prospectus projected the Mariners to finish 73–89, last in the AL West, with the third-worst mark of any AL team.

The Mariner offense was never considered one of the league’s juggernauts, but their August surge pushed them across the 5.0 runs per game threshold for a short time, thanks to an on-base percentage that was in the league’s top third. Center fielder Ichiro Suzuki has hit .351 AVG/.396 OBA/.435 SLG. Third baseman Adrian Beltre has enjoyed his most productive (and least disappointing) season since joining the team three years ago. Right fielder Jose Guillen and DH Jose Vidro, while both light on power, have also both exceeded their projected OBPs by more than 40 points, helping offset a .205/.295/.399 showing from first baseman Richie Sexson.

Had that middle-of-the road offense been backed by a similarly productive rotation, the team would have stood a better chance. Instead, that unit has been the club’s weakest link. Despite playing half their games in a park that depresses scoring, no Mariner starter has an ERA below 4.00. Nominal ace Felix Hernandez began the year with 17 scoreless innings, including a one-hit shutout of the Red Sox in Fenway. But an elbow strain soon sidelined the 21-year-old phenom for four weeks. Since coming back, he’s only had one month with an ERA below 4.73, though his 4.17 overall mark paces the staff.

Free agent addition Miguel Batista has been mediocre at 4.73, while fellow signee Jeff Weaver — last seen pitching the World Series clincher for the Cardinals — has been far worse. Through his first six starts, Weaver was bombed for a 14.32 ERA. After being sidelined for a month by shoulder tendonitis, he enjoyed a productive stretch —14 starts, 3.38 ERA — but a reversion to earlyseason form has pushed his ERA back to 6.05 overall. In all, the Mariners have given 42% of their starts to pitchers with ERAs of 5.74 or above: hardly a recipe for contention.

What kept the Mariners competitive was their bullpen. Closer J.J. Putz leads the majors in Reliever Expected Wins Added (WXRL), a BP metric that measures the increment by which a pitcher increased (or decreased) his team’s chances of winning in each plate appearance; late at-bats in close games or key situations are worth more than those early in the game, in blowouts, or with no one on base. Until recently, rookie setup man Brandon Morrow ranked in the league’s top 10 in that category, with fellow relievers Sean Green and George Sherrill cracking the top 30. Their combined success had the team in a virtual tie for second in WXRL as recently as a month ago, and was a key component in the Mariners’ 31–21 record in games decided by one or two runs. That, in turn, helped the team outstrip their projected record by such a wide margin.

Alas, the bullpen’s collapse has been a major factor in the Mariners’ demise. In this latest 14-game skid, Mariner relievers yielded an 8.45 ERA and 2.3 baserunners per inning, with Morrow, Green, and Sherrill all hit particularly hard. Manager John McLaren, who replaced Mike Hargrove earlier in the season, exacerbated the situation with some curious player usage choices. On August 30, in the bottom of the ninth of a tied game at Cleveland, rookie Eric O’Flaherty put two men on with just one out. Instead of calling for Putz, McLaren tabbed washed-up journeyman Rick White, because — like many a manager — McLaren had pigeonholed his closer for use in saveonly situations. White got one out, but walked two more Indians to send the Mariners to their sixth straight loss.

Still, the Mariners should be applauded for giving a good chase, and for restoring some luster to a franchise that needed it. The danger is that their bid for the playoffs may fool them into believing they’re closer to contention than they actually were this year. Young arms like Morrow and O’Flaherty offer hope that the team can build from within, rather than frittering away money on mediocre free agents — a particular weakness of the more usually inept Bavasi. They’d be better off giving someone else the chance to finish what he’s started.

Mr. Jaffe is a writer for Baseball Prospectus. For more state-of-the-art commentary, visit baseballprospectus.com.