It's because of frieza, plus we were talking about Goku not Vegeta. The power of the explosion was from Golden Frieza in a lower state, Akira Toriyama said after a character goes through a transformation and de transforms there is still power from the form because it is not all the way gone. Evidence for that is when beerus fought goku in battle of the gods, he was fighting him head to head in a lower form after he transformed, that is also why he can become Super Saiyan Blue (Super Saiyan God Super Saiyan)