Do you have any 3rd party alarms on your iPad, which might be set such as Alarm Clock HD or Nightstand? These have wake to music features.
Also, I have heard of people accidentally activating "Hey Siri" while talking in their sleep. I have no idea if that's true or not, since I thankfully don't talk in my sleep. My wife used to, but she hasn't recently, the last time I can recall, was before Siri.
But her name is Sara and "Hey Sara," will often wake up Siri 🙂