why did ham get stringy after cooking

Not all cuts of ham are the same. There are many cuts of meat and some are stringy and some super lean and tender. Think chuck roast versus filet. I'd guess it was the cut not the cooking method but like Pierino notes knowing what kind of ham will provide a more accurate answer.

sounds like the ham was overcooked?

What type of ham and how was it cooked? We need a few more details here.