wait a second

As from the files you find in the game, Cosmog is forced to use its special ability to make Ultra Wormholes appear through immense pressure.

But what exactly is the pressure applied?

From the cutscene, Cosmog’s power looks like this: 

(Source: Pokeli’s Pokemon SuMo cutscene collection)

Seem familiar? Look at the animation for evolution:

(Source: Prof.NoGood’s “How to evolve crabrawler”)

And we do have a previous encounter on forced evolutions!

(Source: PurpleRodri’s HeartGold playthrough: Episode 15)

Tl;dr: Cosmog is forced to evolve and the energy exerted is used to open Ultra Wormholes