When foods are frozen, the fluids in them expand and put pressure on the cell walls. Then when the food is thawed out again, the weakened cell walls break and the fluids escape. You've no doubt noticed that when you thaw frozen meat, the meat gives off liquid. Fruits (and vegetables, too) do the same. I'm not sure that they give off any more liquid than non-frozen fruit would -- after all, they all start out with the same amount, more or less -- they just do it at a different rate. In cooking, it takes a while for the cell walls of fresh fruit to break; the cell walls of frozen break faster, and so the fruit gives up its juice faster.