He is probably just now getting to the point that he is comfortable enough to try and be defensive in that manner instead of ball up. Leaving him alone when he did it the first time probably reinforced the behavior, because no matter what we feel about getting them out, they would prefer to be left alone. "Bite = get left alone? Okay then I'll keep biting." Now you have to reverse this and get him to realize biting does not in fact get him to be left alone. You can hook train him as EL-Ziggy suggested or practice picking him up from behind if you aren't too worried about taking a bite. I don't use hooks on BPs aside from pulling eggs, but many people do because it makes it easier. The type of enclosure you have may make a hook necessary if there isn't a good access point too.