It was the grass.
What worries me is their protocols about shutting down areas that area potentially prone to fire work fallout. They currently shut down behind the castle and wet the ground, but all of New Fantasyland stays open. The grass at both Mermaid and SDMT have caught on fire because off fallout now. And since SDMT is completely surrounded by guest walkways, if the fallout can reach SDMT, it can fall on a guest.
I seriously think they need to think about shutting at least parts of that area down during Wishes. At least on windy nights like last night.