from left field
from (or out of) left fieldfrom a position or direction that is unexpected or unconventional. informal, chiefly North American
The expression originated in the language of baseball: left field is the part of the outfield to the left of a right-handed batter as he faces the pitcher, which does not often come into play.
2000 The Register Nokia…knows that killer apps have always come out of left field—obvious once someone has done them, but prior to that not predictable.