In baseball, the left field is the half of the outfield, beyond third base, that's to the left of the catcher as he faces the pitcher. If you say something is "coming from left field" and you are not talking baseball, you are implying that it's shocking or random.
From the baseball and softball meaning of left field comes the figurative, popular meaning of the term. If someone accuses you of being out in left field, they mean that you're completely unconventional or hold a surprising (or even weird) point of view. This meaning dates from the 1940s, when it originally meant "out of touch with reality."