>>42312369https://swordshield.pokemon.com/en-us/expansionpass/pokemon/slowpoke/"The Slowpoke of the Galar region used to eat the seeds of a certain plant that grew in their habitat, as this was their favorite food. These seeds were in fact Galarica seeds, used to this day as an essential spice for cooking. The Slowpoke in Galar built up particles of this spice in their bodies over several generations, eventually gaining the unique appearance and behavior they are known for today."
"Galarian Slowpoke spend their days wallowing on seashores and riverbanks without thinking about anything in particular. Once in a while they might get a very sharp look in their eyes, but they will soon revert back to their zoned-out expression. It’s thought that this behavior is caused by the accumulated Galarica particles stimulating Galarian Slowpoke’s brain, resulting in it thinking of something tremendous—only for the Galarian Slowpoke to immediately forget what it had just thought of."
I'm not sure where he heard it, but this anon's
>>42307736 claims make a lot of sense too. A bite on the tail and a bite on the head trigger each evolution, respectively, I'd guess.