I will clarify it. They say that he has so much power that he pulses, right? So I reckon that the power inside him was too much, and (I think) that his face cracked for the sake of letting the power out, to prevent Tastidian from exploding.
According to my theory, I say that the crack was a natural phenomenon, not that Tastidian made it on purpose to let the power out.
(like when a tomato has too much juice that it it cracks a bit and squirts juices, but it's natural, and it happened by itself, not made my you or me)