I hate the expression "spasticity", it has very negative connotations these days. Nonetheless, I'll try to answer the question. A stroke can/will damage the brain. If it damages the wrong part of the brain (like one that controls movement, reflexes, motor feedback) then these aspects just cease to work properly. Your mouth, leg, left thumb will not work properly if the part of the brain that "manages" them no longer works as it should.