Toughness is less of an issue with pork than with most other meats, but cuts from the shoulder and leg can still be rather chewy at times. Those muscles are well-used while the animal is alive, and that makes them relatively dense and chewy. Well-used muscles also develop lots of reinforcement in the form of tough connective tissue, compounding the problem. Several ways exist to tenderize even the toughest cuts, with freezing perhaps being the oddest.
Freezing and Tenderizing
Naturally occurring fluid fills the muscle tissues of pork and other meats, which keep the cells plump and your roast juicy. They're mostly made of water, and like any other watery liquid, these fluids expand when they're frozen. That can rupture the cell walls and help break down the structure of the muscle fibers. The effect is well known among seafood lovers, who often favor frozen squid or octopus over their fresh counterparts precisely because of this tenderizing effect. Unfortunately, it's more problematic with larger cuts such as pork roasts.
The downside to freezing as a tenderizer is that it relies on the rupturing of cells for its effect. A more immediate and dramatic result of that rupturing is the loss of fluids in your roast. The juices released by the frozen and thawed pork run out of the roast, either as it thaws in your refrigerator or -- if you cook from frozen -- in the roasting pan. The pan juices evaporate during cooking and give you lots of concentrated flavor, but overall the likelihood of a dry and unappealing roast is unhappily high.
In and Out
If you're roasting the frozen pork, there's a further complication. Heat from the oven only reaches the outside of the roast, and from there it conducts to the inside through the pork itself. Meat, unfortunately, is a poor conductor of heat. By the time enough warmth reaches the center of your roast to thaw it and let it begin cooking, the outer edges are almost certain to be overcooked. When the center reaches a food safe temperature of 145 degrees Fahrenheit, that band of overcooked pork can account for most of the weight of the roast.
If you want to try cooking a roast from frozen, be selective. A relatively flat roast cut from the leg or shoulder is better than a round or football-shaped roast because the oven's heat will penetrate more quickly and easily. Bone conducts heat better than flesh, so use a bone-in roast if possible. If you're not wedded to roasting, consider treating it as a pot roast instead. Cooking your piece of shoulder in broth or sauce speeds the transfer of heat, and the cooking liquid helps keep the outer portions of your roast moist.