It’s not hard to see why Wigan Athletic suffered relegation at the end of the 2012-13 English Premier League season. If a club gives up 73 goals, as Wigan did, chances are it’s going down. In fact, since the league went to 20 teams, every single one that conceded 73 times or more was relegated. But why did Wigan leak goals like a sieve?

I ask this question because Wigan’s then-coach, Roberto Martínez, has been running into defensive problems at Everton, too. It’s also an interesting question because, by at least a few popular statistics, Wigan didn’t look that bad.

Putting aside penalties for a moment, Wigan was 11th in shots for: 11th from open play, and tied for 10th from set pieces, corners, and direct free kicks. They were also 11th best in shots against: 11th from open play, and 15th from dead balls. These numbers didn’t necessarily spell relegation.

So let’s take a closer look, starting with penalties and own goals. Wigan gave up more penalty shots (seven) than any other team except Aston Villa (eight), and all but one was scored. Wigan was also the only team in the league to win just one penalty kick. In addition, the team conceded three own goals. This last number was middling for the league, but in Wigan’s case many of the penalties and own goals were especially costly.

The team dropped nine points as a result of penalties and own goals, when four would have been enough to save them from the drop. The biggest apparent culprit was Maynor Figueroa, who gave up three penalties. I’ve written before that penalties and own goals can look pretty random, so this might have been a spate of particularly bad luck. But in this case, was Wigan’s defense merely weak and desperate?

To find out, let’s look at the other shots it faced. Among those initial numbers, only the shots from set pieces, corners, and direct free kicks looked suspect – and indeed, they were costly as well. The Wigan defense only bested Swansea City in goals conceded from dead balls, 14 to 17. In goals conceded per shot from dead balls, Wigan ranked 15th from top. The biggest issue was set pieces, not corners or direct free kicks; Wigan gave up seven goals from set pieces, again beating only Swansea, with eight.

What about in open play? Wigan’s shot totals there didn’t look so bad, but the basic statistics don’t tell the whole story. Using Opta data, I estimated the goals Wigan would have been expected to concede, based on historical averages, from the shots it allowed. In 2012-13, my model predicted 38 goals from those shots, and Wigan gave up 50. Was it just bad luck, or did something make Wigan especially vulnerable?

It would be easy to point the finger at Wigan’s regular goalkeeper, Ali Al-Habsi. The Omani conceded 38 times, when the model would have expected just 30 scores. Yet Wigan’s backup keeper, Joel Robles, had similarly poor results: 12 goals against, versus eight expected.

Moreover, Al-Habsi hadn’t always put up such poor numbers. In 2010-11, he conceded 32 goals versus 31 expected. In 2011-12, it was 43 to 38. It’s unlikely Al-Habsi was simply getting worse as a player; he was still only 30 years old at the end of that season, and goalkeepers tend to maintain their quality beyond that age.

If anything was eroding, it was probably the quality of Wigan’s defense. In 2012-13, it ranked 16th from top in cards, or 15th with a rough adjustment for opposition possession. It was 14th in fouls committed, or 18th after adjustment. Put another way, Wigan fouled more often than all but two other teams when it didn’t have the ball.

More intriguing is the figure for clearances. Wigan ranked 18th for clearances, or 20th adjusted for possession. That’s right – no team cleared the ball less frequently, given the time it spent on defense, than Wigan Athletic. Perhaps Martínez insisted on playing the ball out of defense on the ground. Or perhaps his defenders, who averaged almost 30 years old at the start of the season (weighted by playing time), were too tired to get to the ball; only Fulham and Sunderland’s defenses were older.

I can start to see a story coming together. Wigan’s defense was disorganized and slow, fouling often and giving away a ton of free kicks on the flanks. When the ball swung into the box, the defense panicked. It also failed to pressure the opposing attackers in open play, allowing them to score more often than expected given the situations when they took their shots. When all else failed, Wigan’s defenders tackled rashly and inevitably gave up penalties. Presto – 73 goals against.

Of course, I’d have to watch a lot of tape to be sure this story was correct. I might start with this video of the 4-0 defeat away to Manchester United. Watch how Iván Ramis tries to play out of defense instead of clearing at 1:23. Then see how Emmerson Boyce, just a few days shy of his 33rd birthday, plays Chicharito onside from outside the box for the resulting goal at 2:05.

Wigan’s offense was nowhere near as bad. Its 47 goals in favor matched Swansea, which finished ninth. In open play, Wigan’s attack actually scored one more goal (32) than expected in my model. But the defense destroyed Wigan’s season and sent the club down to the Championship.

Could the same thing happen to Martínez’s Everton this season? They’ve conceded 33 goals after 20 games, tied for second-worst in the league with last-place Leicester City. Relegation looks unlikely, given that the Toffees are also scoring at a reasonable rate. But if the season ends with European places far out of reach, defense may well have been the difference.