Players will have a boosted chance of obtaining a Shiny Pikachu as part of a special Pokemon Sword and Shield event. From now until August 9th, Pokemon Sword and Shield is hosting a Pikachu Raid event featuring the members of the Pikachu family (and Mimikyu.) As a part of the event, players will have an approximately 8% chance of finding Shiny Pikachu in 5-Star Event Raids. It's noted that, while the player hosting the raid has a decent chance of catching the Shiny Pikachu, other players in the raid have a much lower chance to capture the Pikachu due to Pokemon Sword and Shield's Raid Catch odds.
This is the third event where Pokemon Sword and Shield has given players a boosted chance at finding a Shiny Pokemon. The game previously featured Shiny Magikarp in a New Year's event, and released Shiny Wailord in raids as part of a promotion for the Isle of Armor DLC. Shiny Pokemon are typically hard to find in the wild, so these events represent the best way to capture one of these Pokemon and add them to your collection, especially as you can hypothetically keep jumping into raids or creating them using Wishing Pieces.
This weekend's event was likely intended as a in-game tie in to the annual Pikachu Outbreak Festival that takes place in Japan every year. The event features hundreds of Pikachu "invading" the city of Yokohama and attracts thousands of fans to the city each year. Unfortunately, the worldwide COVID-19 pandemic made the event infeasible for this year, but the Pokemon Company likely wanted to keep the festivities going in some form through this weekend's event.
Players will have until August 9th to grab a Shiny Pikachu through Max Raid Battles in Pokemon Sword and Shield.