Yeah, I noticed that too, sharp bottom left/top right, and blurry bottom left.
That's what you get when you use a Tilt lens like a TS-E.
But it's also what you get when a single element is tilted, or 'decentred'.
Here is a nice write up on the phenomenon.
And you're right, it can happen with a loose element or three. From what I've heard the 15-85 is a bit prone to it, my copy vignettes differently on the left-right sides at 15mm, which is also a sign of decentring. Does yours? Other than that, have you dropped it, or even slightly banged it lately? even worse if the IS was on at the time? and did you use IS for the shot? sometimes the IS elements might not settle to the correct spot when you turn it off.
Another thing to look out for is a worn mount on the lens or the camera, a tiny bit of wear at wide angles can lead to a lot of 'tilt' too.