I managed to catch some of Bills' game, but I didn't pay great attention to his outing pitch-by-pitch.
He was OK the first inning or so, but then he gave up 5-6 hits in a row, starting with a leadoff HR from Phillips. Then, Bills proceeds to give up a few line drives and a flyball double to the warning track to Bruce. He also had a terrible throwing error, which had shown that he was clearly rattled. On paper, this might seem like solid evidence that Bills' velocity is off because he tends to induce more groundballs. He wasn't quite healthy down the second half last year, of course, and to the best of my recollection, he was giving up more flyballs then. Billingsley's velocity was fine in that bad second half, but I'd say his struggles then were more or less attributed to fatigue (hamstring/hip). A pitcher that derives so much of his power from his legs, is likely to have spotty location on a leg ailment or two.
Another thing is he's still adjusting to his new mechanics. A rough start to the year, I suppose, was to be expected perhaps. He's been effective in spots, however, and he can't avoid the big inning, like tonight. Might be mental, might be mechanics, but I'd have to wait and see a few more starts to see if this is indeed a dip in velocity.