I know exactly why, you could easily see it in the game. He was going for the int, he thought the reciever would cut and he could step up and grab that ball, you could see how he played it he didnt speed up till the guy was already passed him. Its a preseason game, and I think Nate was talkin up his INT, and Winfield really wanted to make a play. Same type of thing in the endzone. You could see it in his face afterwords, he gambled and got burned.
This is not, I stress, NOT something he would do in the regular season, the risk of getting burned is too high, BUT since it was preseason I think he just wanted to make a play, knowing that it doesnt really matter, which it doesnt.