Thanks everyone!
@Rithm Gaming
The power ups (score multipliers) come from enemies that you kill. Sometimes, if you kill enemies off screen as soon as they spawn, the multipliers fall down without an enemy nearby. Your ship's gravity sucks in the multipliers as you move close to them, but if you move too fast they get catapulted away.
I'm not sure about the boss going off-screen - he does follow you but he moves really slowly. I think that it might have been one of the heavy enemies that tries to avoid being hit.
The first boss:
