It seems that trent's problems stems from his muscles being too big for his tendons, as reported in another thread. I don't know if this is the case here, but this is the sort of problem that occurs when kids bulk up 'artificially' too quickly, while their body is still growing. By artificially i mean through excercise, either excess running or more likely weights. This is why we have to be patient with skinny kids and not expect them to bulk up overnight.
Again, im not saying this is the case with trent, although it could be, just that this is the type of problem that can occur.