What means "exceptional aspect ratio"?


Suddenly our Twitter Cards started getting cropped every once in a while, for ex: https://twitter.com/nvineberg/status/311474595632922624 . I noticed that Twitter Card docs now have following sentence:

“Images will not be cropped unless they have an exceptional aspect ratio.”

What does this exactly mean, what is exceptional? Is that the cause of our problem with twitter:width/height -tags of 960x1280?