I know it seems too simplistic to some, but it is a matter of natural instincts. Not many, but a handful of guys have it for both sides of the ball. Byrd doesn't. His natural reaction is to hit, not avoid being hit. He reacts to what he sees instead of creating reactions. As Ainge keeps saying on his show, you are who you are. Coaching can only help so much.
I kept hammering this for years on here and BrB was nearly always in agreement. An example was Evan Berry being a natural on offense. He tried to avoid contact and ran to openings, not congestion. The ball in his hands, which were soft and good, did not slow him down. But having him on D was not natural for him, to get in somebody's grill and blow them up. He's not Eric.
Another recent one was M-Teague. He could not do press coverage because his natural instincts were to avoid contact. He would have been a solid slot if the coaches had left him there.
There are exceptions, though. As an example it's different going from D-line to O-line many times. If the players are able to comprehend schemes and blocking assignments it can really work out well due to their natural aggression, leverage and hand skills. Same with moving big RB's and safeties to LB at times. There are several players that were LB's that have converted to RB's due to roster injuries and have done well. Many played both ways in HS and were able to slip right into the role.