The children of Israel did that which was evil in the sight of Yahweh: and Yahweh delivered them into the hand of Midian seven years.
So here was Gideon hiding from the Midianites as he was threshing the wheat because if the Midianites had caught him they'd take the wheat. And so he was out by the winepress threshing wheat and here was an angel sitting under an oak tree.