A RECEIVER turns to catch the ball and it is on him quickly, earlier than he expects.
Or he is not quite there and the ball surprises him, glancing from his hands.
He is moving left and the ball tails right. He expects it to come in fast and it floats, or vice versa.
They won't say it, because it sounds like an excuse from their mouths, but the root of all those dropped passes by Eagles receivers over the first seven games goes deeper than looking the ball into your hands, or concentration, or technique.