"out of left field" (Why "left"?)

[Ittaob at AOL.COM]

As someone else on the list suggested, there's a difference between
major-league and sandlot ball. In amateur baseball or softball play, the
weaker outfielders are put in right field (I know, I was there!) This is
because the good, mostly right-handed, hitters usually power the ball into
left or center field, leaving the right fielders to  worry only about the
poor hitters, who  have only enough strength to poke the ball weakly into
right, or the occasional left-handed hitter.