^Not saying several of the Greats might not have an interest in shutting down the dragon-reagent trade, but I don't buy that the attacks are a GD trying to shut down the trade for a few reasons:
1. The main grievance they seem to have regarding Dzitbalchen's execution is that humans presumed to impose their laws upon one of them and executed him. The autopsy was salt in the wound, but it seems like it was more the indignity of having it performed and broadcast than anything else. Those are the big deals the GD's seem to be making, rather than the use of his body for reagents. Hestaby's speech devotes all of half a paragraph to the fact that Aztlan hasn't handed over Dzitbalchen's remains, and she never even mentions its probable use for reagents. She puts most of her emphasis on the execution itself and the subsequent autopsy/broadcast. And in any case, there's a huge difference between this scenario and the use of talon clippings or shed scales from some dracoform to whom they have no direct relation, so I don't think we can extrapolate any of the dragons' general stance on dragon-reagents based on how they reacted to the Dzitbalchen debacle.
2. They've never, to our knowledge, taken action like this in the past. Dragon-reagent dealing has been going on for decades. If it were really that big a deal to them, I think we'd have seen a major effort to squash it far sooner than this. Great Dragons, by and large, aren't the kind to let something to which they seriously object continue when they have the means to stop it. Of course, it may be that they have been taking steps to stop the practice that we don't know about, which brings me to my third point:
3. Even if we knew for a fact that they were willing to devote significant resources to stopping the trade, this isn't how they'd do it. They'd be utilizing more clandestine and effective means, rather than trying to scare people out of it with very visible attacks on the end of the supply chain. They'd strike at the individuals and corporations actually engaging in the harvesting of dragon-reagents, either by crippling their infrastructure and killing their leadership through non-traceable means, or taking them over, also through hard-to-trace means. Rather than the current string of attacks, we'd see the supply chain suddenly dry up as groups involved in providing dragon-reagents went out of business or disappeared. The current attacks really only draw attention to themselves, and don't do much to address the core ability of humans to deal in the reagents.
The attacks could, however, easily exacerbate tension between dragons and humans, so any dragon might be behind them if that's their goal.