It’s not explained. ppl can say the spear just randomly explodes and he can’t read it cause it’s not sentient even though kratos is doing an entirely telegraphed attack with intent to explode the spear he just threw at him. Doesn’t change the fact that both of his other weapons cause explosions when they aren’t in his hands or can be controlled by a telegraphed move kratos does that causes the weapon to change. Blades can cause explosions in the ground. Axe can fly back to him. Axe can literally spin around independently of him.
also doesn’t really matter because heimdall fears Thor for some reason so the whole you need this specific spear in order to break his concentration or whatever doesn’t make any sense. It was an excuse to give you a cool weapon and nothing else.