Chuck was willing to let the case go until Howard (intentionally or not) implied that Kim and Jimmy became law partners. Then he went into full "Fuck Jimmy" mode, even leaving the house without his suit and letting HHM run normally. He did that out of pure spite for Jimmy.
People keep bringing this up, but in watching and rewatching that scene like 5 times, I see no indication of this. Even if it's true, the speed of the conversation doesn't even allow us to see that he came to that conclusion before Chuck was mentioned.
Here's how it went down, line by line (Not using direct quotes, because it will take too long to type out, but paraphrases:
Chuck hears a noise, asks if it's Ernie.
Howard says it's him. Chuck asks if he saw him, because he expected documents from him an hour ago and he has a round of filing coming up. This is significant because he could be working the mesa verde case that very instant.
Howard tells Chuck that it's likely they'll lose Mesa Verde
"Losing them?! We Just got them! Why? Where are they going?" I'm sure there will be disagreement over level of enthusiasm, but Chuck's emotional state here is shock, surprise, confusion, and interest.
Howard tells him they're going with Kim and that she left.
"Kim left?" He simply looks baffled at this point. "Oh, Howard..." in a disappointed voice. I'm not sure what this means exactly, if he's disappointed in Kim or Howard. "Well...where is she going? Is it Swaikart? Or Reeves and green get her?" Still is maintaining interest. At this point, Chuck is just thinking that Kim is taking Mesa Verde to another firm. To give credit to Chuck's detractors, he looks like he's resigned to this for the moment. But that could be because he sees little chance of him retaking Mesa Verde. He has a lot he can offer over solo practioners, but larger firms can generally do what he does with his team.
Then Howard tells him that she's going into private practice.
Chuck laughs at this. "What? Well, that's completely..." To him, this is pure insanity, and it is a poor move from his limited perspective, given HHM have much to offer her. Again, this shows the disconnect, because Chuck isn't personal with kim, even if he is benevolent. "How is she going out on her own? Why is she going out on her own?" To him, this is neither practical nor smart.
Keep in mind that at no point here does Chuck show anything resembling disinterest. He's not focusing on Mesa Verde, maybe, but he's reeling from surprise after surprise, because this is catching him completely blind sided.
Now, Howard tells him she's pulling her resources with jimmy.
"Kim and my brother. Partners at law" Howard doesn't correct him, and while I think people overplay this as a means of manipulation, he's definitely letting Chuck know Jimmy is in there some where. "The man is Svengali" Svengali is...well, look up his wikipedia page. Again, he's not wrong, Kim is going out on her own because of Jimmy talking her into it, but he's putting it in the worst light possible.
Howard now moves the conversation back to Mesa Verde. Chuck seems to not be paying attention, he was thinking to himself for a minute there, which howard gets his attention again. Howard asks for help, at which point Chuck springs to action.
So yeah, no one denies the likelihood that Chuck was encouraged by the prospect of taking away a client from Jimmy. But like many scenes, this has multilayered motivation. Is spite a part of it? Sure. But he also believes he has a duty to watch out for clients, which he immediately knows that Jimmy will not be able to handle right. He's also showing concern for Kim (Patronizingly so, but still). And he''s just being an HHM lawyer. It can't be argued he was at any point disinterested. He was shocked, listening, and demanded answers as to what the hell was going on.
How much of it was spite, how much was genuiene concern for clients, and how much was just being a typical lawyer is debatable, but I think it's safe to say that chuck didn't go from zero to 100 at the mention of Jimmy's name. More like 80 to 100.
...I just realized.
Chuck's going to show this to Mesa Verde. Not the police.
He'd put it on youtube if he could. He'll show it to everyone who will listen.