What Are Surprise Dynamax Battles?

When traveling through the Isle of Armor, you will eventually be thrown into what is now being called a Surprise Dynamax Battle with Vespiquen while searching for Sticky Honey with Hop. This feature continues on the Crown Tundra, as there are two different Surprise Dynamax Battles to be found. The first is with Regigigas by activating the Empty Pokemon Den in the Giant's Bed, done so by having all five of the Regi Legendary Pokemon in your party. The second is with Greedent, done by shaking the giant red tree in Dyna Tree Hill after the Galarian Legendary Birds flee it. Regigigas has the chance to be captured, while Greedent does not (but you will be rewarded greatly with berries for defeating it).