"Do you think pressure from new Chromebooks is partly responsible for the downward movement in pricing?"
Personally, no - not in the slightest. I don't believe that Chromebooks are even a blip on the radar for the MacBook Air. They aren't even on the same planet. Now, if you want to ask if there's any pressure from Intel-based Ultrabooks, well, perhaps. But that hasn't had much impact on Apple's pricing in the past, so... the most likely reason may be to juggle pricing due to new models on the horizon. I can't see Apple suddenly getting worried about other platforms like that.