Last Friday night - it was just last Friday night! - the Mariners lost to the Indians when Brandon League allowed a walk-off home run to Travis Hafner. On the one hand, it was nothing exceptional; League had taken the loss in his previous three outings as well, and we'd seen the bullpen have some earlier struggles. But on the other hand, the decisive event was uncharacteristic. Hafner's home run was only the fourth homer hit against the Mariners bullpen all season, and they haven't allowed another since.
The Orioles' bullpen has allowed 26 home runs on the season. The Rangers have allowed 17. The median is 11. The Rays have allowed five. The Mariners have allowed four, which is the lowest total in baseball. They also have a HR/FB of 4%.
For the sake of gaining even more perspective, relievers Huston Street and Josh Rupe have each allowed more home runs on their own than the Mariners' entire bullpen.
Unfortunately, you're mostly all smart enough to understand that this isn't going to continue. Eventually, more fly balls are going to start clearing the fence. The lowest bullpen HR/FB a year ago was 6%. The year before that, it was 6.7%. Even with offensive levels down, one expects the Mariners to finish the year around 7-9%, meaning more home runs are on the way.
But it isn't all bad news. For one thing, the Mariners bullpen generates more groundballs than any other AL bullpen, meaning there are fewer fly balls to pose threats. And more significantly, despite the home run luck, the Mariners' current bullpen ERA is a good deal higher than its FIP and xFIP, due to some unsustainably bad timing. This is what happens when opponents bat .239 with the bases empty and .290 with runners on.
So what we should see going forward is that the unsustainably good things and the unsustainably bad things will more or less balance out. One also figures that the re-introduction of Shawn Kelley should provide a boost, but the bigger message from this post is that we can expect the Mariners' bullpen to continue to be inconsistent in the future, only in a slightly different kind of way.