https://www.nytimes.com/live/2022/01/03/technology/elizabeth...
https://www.nytimes.com/live/2022/01/03/technology/elizabeth...
This might be because Saturday in Hindi and other Indian languages is “sunny-var”, and the day of the week you are born on is a common nickname, in conjunction with “sunny” being an easy to pronounce and common English word.
I feel cheated. I got my nickname because my baby sister couldn't pronounce my name correctly.
From what I understand, the reasons for picking nicknames are quite random. As another commenter mentioned, people named Sunil or Sanjeev go by Sunny sometimes, although other times it is just picked out of the blue, like with Bollywood actor Sunny Deol, whose real name is Ajay. His brother Vijay uses the screen name Bobby, by the way. Random.
Sometime people named Sunil, such as the cricketer Sunil Gavaskar, use Sunny as a nickname.