There are two questions here:
First about staying low. It is all about coaching. It is also hard for a 6'7" man to get low, much less stay low. It's simple ballistics. But it can be learned with practice. It's a balance thing. But it's not like you have to run 10 yards hunkered over. You actually fire out low and get under your opponent and then stand him up (it's the same hip thrust as in the power clean, your power comes from your base), and then defeat the block. So it's a fire out thing, and the guy on the LOS who gets the lowest (and stays balanced on his feet) usually wins the down.
This is a real problem with the extra big HS players. They can just dominate w/o learning technique, and many HS coaches just let them. Then they struggle at the college level.
Second, IMO, AT Barnes is very athletic but a good bit undisciplined. He has over run many a play. With only two true LB's in this D, and with guys like Sed, Jefferson, Jackson, Sylvester and now Burnett, it is no surprise to me his PT has gone down. He seems to be a great young man, though, and I hope he gets back in the 2-deep.