Could have been for a special event? There's been a few times when I worked at Epcot that we would see fireworks from the park taking place at unusual times/places.
Last October I was staying at the boardwalk villas in a room facing the swan and dolphin and the had fireworks too. I think it was either a special event or part of their food and wine event.
I just came home from WDW on Thursday night, and I had stayed at the Swan since Sunday. I did not see this particular fireworks display; and as others mentioned, it was probably a special event.
What I did see from my hotel balcony was most of the fireworks show from DHS at 9:30 each night. That was fun.