The Eagles have been connected to corners in the past and Johnson was an All-American. It's a natural fit, especially if the Eagles end up trading Asante Samuel. This gives them plenty of depth in the secondary.
SB Nation's Eagles blog, Bleeding Green Nation, explains the pick:
At 6-2, 200 pounds with a big 33 ¼ wingspan, Johnson has rare size and strength for a corner, which will be important as WRs get bigger and stronger not just in the NFC East, but all around the NFL. And while a knock against him is that he didn't play elite competition in the Big Sky Conference, he did dominate the competition he faced. What he lacks in top end speed, he makes up for with good ball skills and a polish that's rarely seen from smaller school prospects.