Sourdough focaccia rose way too much

My wife and I are brand new to sourdough and began our first starter 5 days ago. We read about someone who made their own sourdough focaccia with a 6 day old starter, so we decided to have some fun and try with our 5 day old starter. Our started doubled in size during the bulk fermenting in about 5 hours. After we moved it to the pan, it proofed until it filled the pan in only 2 hours. We baked it at 425 degrees F (220 C). Does anyone know why it rose so much? Shouldn't it be flat? Thanks!