I can't work out how to post a picture to show it, but I've noticed that every once in a blue moon there is one patch of shaking grass while using the Pokeradar on BDSP that appears to be larger than the others. Is there anything special about them? It doesn't appear to be related to shinies.
Larger Shaking Grass has Hidden Ability Pokémon in it. It has a 1/128 chance to spawn.
Source 1, 2, and personal Experience.