rolling (ankles) in the deep

On not so random occasions, my ankles roll while walking even on flat, level surfaces. In fact just did it during my lunch break. I know the difference between the times I can just shake it off and when I should call it quits and let it rest. I know how to treat the varying degrees of pain. My questions are 1) Does anyone know what causes it (weak muscles/ligaments/tendons; which ones in particular), stupid brain that momentarily forgets how to walk, etc)? and 2) What exercises can I do to reduce the number of times this happens by strengthening the joints/muscles/whatever?

ETA: When it's a bad roll, my knee and occasionally the hip will also feel sore and weaker. I obviously want to learn how to stop these before I get old and start breaking bones from falls that will inevitably happen.