It happened the same night, that Yahweh said to him, Take your father's bull, even the second bull seven years old, and throw down the altar of Baal that your father has, and cut down the Asherah that is by it;
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.