I think there is a word that means something is presented or said without context, like a statement that appears random. "Leftfield statement" comes close, but I think there's something more concise.
Some people malapropriate "non sequitur" for meaning this. (I'm basically looking for the word that means what those people think "non sequitur" means.)
I'll give an example of what I would describe with the word we're looking for:
Someone on Twitter posts the message: "Oh, not this again…”
No context (e.g. in previous tweets), completely leftfield.