It's not actually very steep, it just looks like it because it's been at (effectively) zero for a long time.
If you look at the Java one for example, which is on a long downward trajectory, you'll notice that they've scaled the graph to fit the data. The 2-3% rise that both languages had in 2011 looks much bigger on the Objective-C graph than it does on the Java graph (where indeed it's just a small wobble on its long, slow descent).
In fact all of the top languages seem to have steeper slopes (often in both directions, and on multiple occasions) than the Obj-C one.
If you look at the Java one for example, which is on a long downward trajectory, you'll notice that they've scaled the graph to fit the data. The 2-3% rise that both languages had in 2011 looks much bigger on the Objective-C graph than it does on the Java graph (where indeed it's just a small wobble on its long, slow descent).
In fact all of the top languages seem to have steeper slopes (often in both directions, and on multiple occasions) than the Obj-C one.