His lateral movement is very violent and choppy, not smooth at all. It probably comes from running track so much. You don't train on lateral movement in that sport and it can throw off the lateral mechanics for some guys. I'm sure if he played ball and football together growing up he would have better lateral movement. Aaron Rodgers would make that same scramble outside the pocket and it would look smoother than RG3's.
Its because he runs on the field like he's on a damn track.
When defenders start closing in he opens up his stride instead of shortening it. He has major trouble in short areas. Same way he kicked his leg up in the air against Baltimore when he first hurt his knee. Same way he did now and against Seattle (the 1st time he tweaked it)
I noticed it in college and was concerned when he did none of the quickness drills at the combine. Everyone knows he's fast...but his change of direction skills are suspect.
Andrew Luck "moves" better on the field that RG3 as well.