Well, a sweet vinegar taste is pretty definitive- sounds like you maybe got some acetobacteria in there. The "how" is difficult to pin down, except by you. Certainly sounds like you've got sufficient experience to brew correctly, but as you know, there are a number of places where infection can sneak in. You only mention opening one bottle, I'd sure open a sampling before I tossed the whole batch.
“Malt does more than Milton can / To justify God’s ways to man”
-A. E. Housman (1859–1936). A Shropshire Lad , 1896.