WitherBoss attacking when not supposed to be

When I summon a Wither Boss normally, (place the soul sand then the skulls) the Wither Boss will suddenly attack the living mobs nearby even though the health bar is still filling up. First pic I freshly summoned the Wither Boss. Second pic the health bar is still filling yet the Wither Boss has started attacking but not flying. Third pic shows the big explosion as normal, and the Wither Boss can fly as normal.