Godzilla was on a meditative retreat in this movieWhen the survivors find him in the cave, they think he's sort of dead. That is incorrect. He was actually in a deep trance. After he fell into the ocean during Invasion of the Astro Monster he swam to Devil's Island for some R&R. On his way over, he gave Ebirah a Tail Slap and moved into the cave. After the boys woke him up, he was angry and thought Ebirah was responsible, so he went out and punched him around a bit. After scaring away the soldiers after Dayo, he calms down, sits down and goes back into a trance. The humans mistake this for sleep. Then he is interrupted twice more by the giant bird and then the planes. By then he's like "Screw it, I'm going back to wrecking stuff."
WMG / Ebirah, Horror of the Deep