Combination of things, I think. First of all, he had an excellent week at the Senior Bowl, which caught plenty of attention. He showed natural ability to catch with his hands, and showed the ability and willingness to make catches all over the field.
Then, he surprised at the combine when NFL Personnel folks had a chance to see him up close and personal. The Ourlads people commented that he was:
'Bigger and thicker in person. Looks skinny on tape.'
In their most recent Newsletter, they listed him as the best 'run after catch' WR in this years class.
From a measurables standpoint, I'm pretty sure he had the longest arms, and some of the biggest hands of all the WR in the draft this year, ran a top-notch short shuttle (the indicator for change of direction speed), and had one of the better vertical jumps as well.
NFL Teams have been personally interviewing prospects over the past couple of weeks. I think it's likely the scouts who went to the Senior Bowl and Combine and took note of this guy had a chance to show him off to the rest of the personnel department for their respective team, and the more they see this kid up close vs the perspective they might have drawn of him from seeing tape highlights, it dispells any worries they might have had regarding his physical attributes. At least, that's my guess.