Gastropods sometimes display an incomplete torsion of their shells when they are not supposed to grow that way. I always thought these incomplete torsions were caused by the veliger receiving either physical trauma or some sort of attack by another organism, but I'm not sure. Why do gastropods sometimes display incomplete torsion of their shells? [/size]This might be one of those questions to which no one in the entire world knows the answer. I've often been accused of asking those.

You are correct - it is most unlikely a definitive answer will be known. That said I am sure you are correct that trauma or foreign objects (eg fine particulate matter) are most likely to be the cause as they will alter the shell development.