Bush has seen improvements, though, in a number of categories that might help us understand what's going on.
His HR/9 is down from a career-high 1.50 to 1.03, opponents are hitting him for a lower average overall (from .289 to .276) and on balls in play (.324 to .297), he's improved his GB/FB ratio from 0.77 to 1.10, and he's allowing less line drives.
I think to find the biggest reason for Bush's good results lately, though, you have to look at how deep he is (or isn't) pitching into games. Remember how we'd always criticize Ken Macha and Ned Yost for leaving Bush in the game far too long, just trying to get him through 7 innings?
That's not a problem this year. If you give Macha credit for one thing this year, it should be for being proactive with Bush and taking him out at the first sign of trouble, no matter how manageable his pitch count looks.
Bush's pitch counts in his starts since the debacle in Minnesota: 102, 86, 107, 96, 98, 99, 101, 90. That's right, in his past five starts, Bush has thrown over 100 pitches once. This isn't something that just popped up after that night in Minnesota, either -- before that start, Bush had pitch counts of 96, 77, 96, 87, 108, 95, 102, and 99.
So, when we're trying to figure out how Bush's pitching lines always seem to end up looking better this year, maybe it's as simple as Macha taking him out of the game before he starts hanging curves and grooving his 86 MPH fastball. I'm sure it's not the only factor working in Bush's favor, but it certainly can't be hurting him.