Tainted Area

This is speculation but it's also possible it's because of Tory Ravine. A few times through G15 they mention how Admiral Owen restricted access to Tory Ravine without a publicly announced reason and it is also known that Tory Ravine is based on Tory Island, the home of Balor in Celtic Mythology. Also, if you look at the streams in Beach of Scathach, you notice that the ominousness is coming out of the direction of Tory Ravine, not the ocean. Although, if we ever see Balor ingame, it would probably be in a flashback or some revival since Lugh, Balor's grandson, kills him and Lugh is obviously dead.