FiveThirtyEight thinks they're probably juiced too, great article (they don't have definite proof but their is strong evidence):
Quote
But although our results are suggestive, they’re not conclusive. To make a more compelling case, we’d need to conduct additional lab tests and start doing dissections, although those might also leave us looking for answers. (We’re unlikely to slice open a baseball and find a Super Ball inside.) According to one hitter we spoke to, who’s played in both the majors and Triple-A since the homer explosion started, the juiced-ball theory “seems to make sense and there is no doubt in my mind that there is a difference in the balls [between Triple-A and the majors]. I just wouldn’t want to get so tunnel vision about the balls [that] we could be missing a bigger issue.”
He’s right to reserve judgment; one thing we’ve learned from this mystery is that even when armed with big data, we can’t declare every case closed. But the deeper we dig into baseball’s record home run rate, the harder it is to believe that it doesn’t have something to do with the ball.
http://fivethirtyeight.com/features/are-juiced-balls-the-new-steroids/