It's heavily implied that The Scarecrow came to life when the Munchkin farmer who made him first painted his face, but we never learn WHY he came to life.

Ruth Plumly Thompson attempts to answer the question in her sequel books with racism, but I refuse to consider that canon. You can look that up yourself if you're curious.

In the 1903 broadway musical, Dorothy accidentally brings The Scarecrow to life with a magic wishing ring that the good witch Locasta gave her. I almost made that canon to Yellow Brick Ramble, but didn't when I realized that'd erase his backstory with the crows.