Strange loading screen when MBP battery dies and reboots
This feature is called Safe Sleep. Apple notebooks will keep the RAM contents alive (sleep in PC jargon), but write a copy to the disk (hibernate, in PC jargon) at the same time.

If the battery runs out while the computer is sleeping, when it wakes up, it needs to read the RAM contents from disk, and thus you get the progress bar you are seeing.

Apple doc about it is here