The hand of Midian prevailed against Israel; and because of Midian the children of Israel made them the dens which are in the mountains, and the caves, and the strongholds.
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.