Why did I have issues scraping off a popcorn ceiling?

My house was built in 1971. After removing a think layer of popcorn, I found cardboard behind it. Does anyone know what that is? Is it safe to just paint over it?

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Edit:

JACK has the correct answer.

I'd just like to add that the reason why I scratched all these holes is that I didn't spray enough water onto the ceiling, so the bound between the popcorn and the ceiling was too tight for me to use a knife to separate them.

After learning my mistake, the rest of the ceiling got scraped off very smoothly.