I think it suggests that the thing is unprompted, and nobody asked for it.

It used to make sense, like… people that need no provocation to go into something they won’t shut up about, or things like that. Lately it has just stopped being used in any way that actually makes the “Nobody:” part matter.

It's a variation on "nobody asked for this", "nobody asked you", or "you know what nobody wants? Cars with DRM!"

I'm sure it's intended to indicate that "nobody asked for...", but personally I find that almost all memes that use it would be better off without it. It's honestly kind of annoying.

Other memes might present some fictional dialogue as setup and context before the image or final line or quote or whatever at the bottom delivers a punchline.

"Nobody: " could be read as "Nobody said nothing". Sometimes further exaggerated with extra lines of hyperbole about how absolutely nobody said a single goddamn thing.

Which, in theory, serves to emphasise that the punchline comes out of left field. An absurd non sequitur with no setup, no context, and no-one asking for it. It's not a response to anything, it's just an inexplicable bit of nonsense that came out of nowhere.

But also it's really easy to tack that on to just about anything that would otherwise have just been a mildy funny image, in a lame attempt to heighten the comedy of it. So it gets overused.

At least I suppose they’re not memes that start “someBODY…”

When the "nobody:" part is blank, it means that nobody said anything, or nobody did anything.

As others have suggested, the meme means that some would say or do things without any trigger or nobody asking for it.

I'm pretty sure it's implying that nobody asked at all; like, literally no one is asking about it. Knowyourmeme explains that it's, "...a phrasal template used to mock people who strive to attract attention and tend to provide unsolicited opinions, as well as for observational humor."

Ok I get your question but that's a solid joke about Optimus prime