At least with Rolando McClain I can see a few good arguments for him retiring early:
#1: He signed a rookie contract under the old CBA, meaning that he already has several million dollars in the bank. McClain was paid something like $22 million in guaranteed money before taxes. No reason to keep playing just for the money, especially on $700k contracts. If money was the goal all along, getting out now makes sense as the added money isn't worth the long term health ramifications and costs.
#2: He played for Oakland. That's going to end your hunger to play the game real quick. Jamarcus Russell was in a similar type of situation and just decided it was better to walk away.
In the case of Young, he wasn't rich, seemed to be coming into his own and had all the reason in the world to be on his best behavior and now he's out of the NFL and now maybe into a prison cell. I think McClain made a defensible decision to retire given the money he has and that he clearly doesn't have a heart for the game anymore. Titus Young on the other hand, is a nutjob.