When I saw that little tidbit flash across my Twitter feed I was certainly surprised, after all Danny Duffy had just gotten done turning in six quality innings against a potent Tigers lineup. While Duffy has certainly struggled to a 4.85 ERA and his sometimes sketchy command has led to a propensity to run up his pitch counts quickly, he's also shown flashes of the type of dominance people expect to see more of in the future - like striking out more than a batter per inning during his last four outings.
In a rotation that ranks second to last in all of Major League Baseball with a 5.06 ERA (The Cubs are the worst at 5.23), Duffy would seem to be the least of the teams concerns.
But it would appear that Duffy's demotion had nothing to do with his performance, rather the team is simply taking advantage of an opportunity to keep him working. While Duddy's next scheduled start would've fallen during the All-Star break, by sending him back to AAA Omaha, they can keep him on his regular schedule without it effecting the teams rotation depth.
It also gives the team an opportunity to carry an extra reliever, and they've chosen to bolster their bullpen by giving 27 year old lefty Everett Teaford another opportunity. Teaford worked in 14 games earlier this year, notching a .284 ERA, but posting horrible peripherals, including a negative K/BB ratio. In all likelihood the poor K/BB numbers are an aberration as he's consistently posted strong metrics in the Minors, including an 8.13K/2.61BB ratio in AAA this season.
Duffy's first regularly scheduled post-all-star-break start would've been on the 19th, and it's likely that the team will keep Teaford (or another player) on the roster in Duffy's spot until the day before that start before calling their talented young lefty back up.