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We know Eternatus is the evil legendary, it's been hinted at in the game trailers (the doggos reveal one) and in the confirmed leaks. And we know (or theorize) that the giganta/dynamaxing is not the greatest thing, and is probably actually bad. Dex entries for Pokemon like Gigantamax Meowth talk about it ruining skyscrapers, but all the dynamaxing we've seen is in sanctioned league battles or raids in the Wild Area. This suggests that wild, random dynamaxing was a problem until recently, or mayhaps is even still a problem in plot we just haven't seen yet from the limited trailer footage.
An interview had the staff talking about how a lot of inspiration for Galar came from old UK/western legends about giants shaping the land. This is visible in the trailers as being pretty heavy into the lore with the huge hillside petroglyphs and the fact that the raid spawn/gathering points resemble the Giant's Causeway. The pokemon getting BEEG shapes the land, or at least is meant to even though they're supposedly essentially holograms in the league battles, the graphics of heavy debris kicking up when they land/attack suggests this is not entirely true.
Eternatus is waking the ability to dynamax within the land itself, likely trying to return the region or even the world to a more primitive time by using giants to shape it. It's an old god, and it was alseep. Now it's not, and it's piiiiiissed.
