You do not say what species your tarantula is or under what conditions it loses its legs but I'm assuming this is a molting issue? Species such as the Goliath Birdeaters the T. Blondi and the T. Burgundy are just notorious for molting problems, but any species can have molting issues if not care for properly. Do you have enough humidity? Are you feeding your specimen mice because that's REALLY not a good idea. Not because they cannot over power a mouse, they can, (although the mouse fighting back should be a concern) its the calcium in the bones. Tarantulas don't have bones and too much calcium in their diet causes calcification of their exoskeleton. Huge cause of molting problems.

Because I'm black.