What happened to this Miami Marlins reliever?

The Miami Marlins recently made a change at closer, replacing David Robertson with Tanner Scott. Why are The Fish in a position to improve the bullpen again? What happened to David Robertson? How did he go from being an elite closer to being a complete bust? Is it his mechanics? What went wrong for him? Let's take a look "under the hood"....

What went wrong for Miami Marlins relief pitcher David Robertson?

David Robertson entered this season with a career 2.89 ERA/2.92 FIP, with 11.9 K/9 and 3.7 BB/9 in 731 games and 739.1 innings pitched. This season has been a tale of two different...well seasons. He had a strong 2.05 ERA/3.58 FIP, with 9.8 K/9 and 2.7 BB/9 in 40 games and 44.0 innings pitched. For the Miami Marlins, he has an 8.18 ERA/5.54 FIP, with 9.0 K/9 and 4.9 BB/9 in 11 games and 11.0 innings pitched.

I was concerned with Robertson's FIP before the trade, but even if he simply matched the 3.58 mark, the 8.18 ERA could not have possibly been predicted. His strikeout rate clearly went down from 9.8 to 9.0, but that's not that big of a change. The walk percentage is definitely a concern, going up from 2.7 BB/9 to 4.9 BB/9. Looking deeper there don't seem to be any changes that I can find, and the small sample size does make it difficult to judge as well.

One of the things that maybe an issue here, is Robertson is already 38 years old. It's possible that the combination of age-related decline and fatigue may be causing these results. There's also the pressure of being in a playoff race. The New York Mets are not going to the playoffs this season, and Robertson may not have been feeling as much pressure to perform. He's now in a playoff race, and that may be putting extra pressure on his shoulders.

There's more potential problems for David Robertson on the Miami Marlins. Is it possible that his control being off is due to some mechanical changes? Or is it just plain old fatigue and decline? Regardless of the cause, he should not be pitching in high-leverage situations or maybe even at all for The Fish going forward.