I don't think LeBron is scared, exactly. I do think he's still not over the way he was sent packing during his Cleveland years. It's typical of elite athletes to nurse a past failure or slight so it can be used as motivation. And it's typical of spoiled brats to spend years wanting to shift the blame: "I wasn't the jerk for blaming that elbow injury... they were the jerks because of.... REASONS!" He couldn't break the Celtics then, but he can throw a little extra dirt on the ubuntu memory.
However, I don't buy his stated reason that he was defending his teammate. He's generally good about taking care of his teammates in public and by most accounts in private, too. I'll give him that. But in this case, no one had said anything about Ray Allen (other than "championship saving shot maker") in a news-cycle eternity. What did get said about him by Pierce and Garnett a year earlier wasn't that bad; it was more "I'm mad at him" and less "let me tell you about the many ways he's a horrible person." Allen's own response was to laugh. Ray Allen, though at the end of his career and reduced to role player, isn't an insecure lowly bench player without the ability/audience/credibility to stick up to big bad Pierce and Garnett all by himself.
So at the end of the day, I think LeBron's real reason for bringing it up was for publicity. Sure, LeBron already has plenty of that. He's defending champion X2, generally considered to be the best player in the world. He's not hurting for news coverage, but he's aware of his image and figures he can always use more attention in his stated quest to be a billionaire and mogul. When he brings up Ray Allen, his fans say "We love LeBron for defending his teammates! We love LeBron for telling those jerks Pierce and Garnett where to go!" And his haters say "We hate LeBron! A trade and free agency are not the same!" Either way, LeBron's name is on everyone's tongue.
Pierce and Garnett responded for the same reason. Their haters will say "they're still the most annoying studio gangsters ever, all talk and no substance!" Their Nets fans will say "glad they're here to help give the team the grit to stand up to LeBron!" And their Celtics fans will be delighted by KG's mumbling about "Celtic business" and Pierce's oh-so-innocent reminder that he didn't choose to leave. Either way, their names are on everyone's tongue.
And what about Ray Allen? The guy supposedly at the center of it... the guy who openly talked about crafting his image with nice suits... the guy who was the media's favorite all through his career and constantly made sure he was portrayed as both highly intelligent and a darn nice guy... is silent. He doesn't want to switch over to a controversial image at the tail end of his career when he can't play much anymore. He wants to remain the guy who got the Onion writing satire about "NBA star lauded as decent human being." So he doesn't criticize his former teammates; he certainly can't criticize his current teammate.
The NBA is a cult of personality. They're all just marketing their brands in preparation for the new season. If a little honesty about old grudges slips through, that's incidental.