I'm tempted to label this "time travel shenanigans." In fact, when this particular scene was released several months before Peace Talks came out, I read the description and the term "cornerhounds" and immediately said "Hounds of Tindalos in Dresden?"
When Butters opens up the very next chapter with "Holy crap, Hounds of Tindalos are real?!" It solidified my opinion that time travel would kinda have to be involved in some way, else it'd probably be a different type of Outsider. It feels too much like an obvious clue/reference.
My argument:
1. Justine is not the summoner, because she needs Harry to live long enough to escort her to Demonreach, Nemesis's real goal. It does not suit her plans to try to kill Harry, especially not when she knows exactly how to manipulate him. (Literally just "being a woman asking for help" is enough 99% of the time for Harry). She also does not demonstrate any wizard (or even warlock)-level talent in any previous book. Thus far, the only non-wizard/non-warlock we see using mortal-style magic is Thomas, when he uses the locator spell Harry taught him (offscreen in... White Night, I think), and Butters, when he makes a circle in Dead Beat (I'm excusing all of the single-use magical items he uses in his pseudo-Batman impression, though maybe there's more than a little talent required to make those). My understanding is that you do need legitimate magical talent to summon an Outsider.
2. Time and effort are required to summon one, and even then it is dangerous for the summoner. Harry does a full ritual whenever he does anything of the sort. In Proven Guilty, there's a definite spell that builds energy to summon the Fetches. I don't think Justine had the time, and Eb is onscreen long enough before they show up that I don't think he did it, either.
This leaves the wizards we know are in Chicago, but aren't in that scene: Carlos, Chandler, Listens-to-Wind, Wild Bill, Yoshimo, Martha Liberty, and Cristos. Potential additional summoners (those who have enough power) include: Molly, Mab, Titania, and Sarissa. I don't think the Queens are mortal enough anymore to summon an Outsider, but Molly and Sarissa probably are. Odin and Ferrovax are out for the same reason. The Archive could do it; so could Marcone, I presume, though I find them unlikely.
Based solely on character history, I'm going to eliminate several: Listens-to-Wind, Wild Bill, and Yoshimo. LtW doesn't strike me as the type who'd do business with things that upset the natural order, and Wild Bill and Yoshimo get carried off by Drakul, presumed Dead or Turned.
That leaves us with Carlos, Martha Liberty, Cristos, and Chandler. It's possible that Carlos did it, and there is one interesting potential possibility: Carlos was tracking Harry at the time, and knew where he'd be.
Martha Liberty is a relatively unknown quantity. She's only had a handful of lines in the series, most of them supportive of Harry. She's also an odd one among Wizards: she lives with her descendants.
Cristos is pretty obviously a bad guy, but he also fights pretty hard in Battle Ground. Earth magic, it looked like. It's possible he's a stooge.
No, of the wizards in Chicago during Peace Talks, IF the cornerhounds were indeed summoned, my money is on Chandler being the culprit. We know a handful of things about Hounds of Tindalos that may (or may not) apply to cornerhounds: they are drawn to time travel. Chandler's specialty is messing with time. It may or may not have been intentional, but Steed is the only wizard explicitly stated by Jim to have an affinity for time-related magic, and now we have time-travel-related Outsiders show up.
3. Related to my last point, I think we're left with only two possibilities: A) the cornerhounds were indeed summoned, to kill Harry. My bets are on Chandler being the one responsible, though I concede Justine is possible, just not with any of the skills we can confirm she possesses (meaning it'd be a thus-far unseen ability of hers or Nemesis's). I would imagine those particular types of Outsiders would be the first ones to answer Chandler's summoning, mostly because of the aforementioned affinity with time.
The other possibility is the one I find more likely: B) the cornerhounds are there as a side effect of time travel happening behind the scenes. Either it's a time travelin' Harry on an adventure we'll see detailed in a future book, or it's Chandler. Remember, we have no idea what happens to Steed when Drakul tosses him through a portal. He may have used time travel magic to escape, or that might have been what Drakul did to him in the first place. Based on Odin's explanation of time travel (and the ripples on the past) during Cold Days, it's possible that someone bending the laws of time a few hours after the attack (during the Drakul fight) might have an impact several hours earlier, so I think it's possible that the cornerhounds showed up earlier due to such a ripple effect.
