Hands are actually pretty easy to injure, and modern medicine is the reason most of us get to keep them all our lives. I've known enough farmers and construction workers who are missing digits to assume a significant number are likely to be from injury in agricultural or hunting contexts. Frostbite would be another easy source of injury depending on climate.
While I could see a possible religious practice coming out of reverence for injured hands contributing too, this seems like the age old archeology practice of assuming anything is intentionally done for religious reasons if they don't have a neat and tidy singular explanation.