I'll take a shot at it, as I continue to rack my brain for what it's similar to...

"Oh, when did you become so honorable?"

Any other thoughts?

How come.....
und irgendwie weiter

Flatternscheu, ich denke, dass ist nicht gemeint...

#1. geht gar nicht

Who did you have to kill? (Nicht Muttersprachler)

What did you do to earn that hono(ur?

evtl.

Ich habe es nie gehört :/

Ach, "how" statt "when" o.O

To what do you owe this honor?

"Oho, and what entitles you to this honour?"

$5 is best, so far. #7 misses the point and doesn't fit here. It's actually part of a greeting: And to what do I owe this honor?

Here maybe:
Oh yeah? And who'd you have to blow? (granted, that's a bit nasty)
How did you manage to do that?
Really? They couldn't find anyone else?

I guess it depends on how nasty/sarcastic you want to get.

#7 misses the point and doesn't fit here.

You don't say. It's actually a fairly faithful translation of "wie kommst du zu der Ehre".

It's actually part of a greeting: And to what do I owe this honor?

Well, it can be used as a greeting, of course: To what do I owe this honor? = What brings you here?, but you'd probably only use it sarcastically or humorously these days in this context.

I wouldn't use it in this context at all.

In this context = as a greeting. It's fine when used literally.

If you made it 'To what do you owe that honour' it might work. But the honour is not the honour of meeting the speaker, is it, so 'this' doesn't work for me, either.

Suppose though thaat the speaker is looking at a paper with "the honor" on it, pointing to it, and talking to the other asking the question with "this".

A little specific, but "this" can work.

And ignoring what the other person just said. Quite specific, yes :-)

If you're trying to be sarcastic to begin with, I see no reason that somebody couldn't do this to be a little meaner :)

"hmmm? Did you say something? It must've been the wind..."

I'm not going to argue this vs. that (or honor vs. honour, for that matter), although the latter (... owe this honor ...) does seem to be more idiomatic, or at least more widely used (in A.E. only for all I know).

who had such this bad idea?

all you need now is a toga to go with the laurel/s...