The answer is, it’s a bit of both.
The Alliance’s stated goal, per the NPC Dialogue in the Alliance quest, is to destroy the supplies. This does mean destroying the caravan wagons. This is further reinforced by the Alliance NPC death quotes.
But, the 7th Legion was attacking Vulpera on the Horde side of the quest. Here’s a video:
https://youtu.be/yDRNp9N49QA?t=235
I’m linking to a particular timestamp because there are actually a few vulpera NPCs. The 7th Legion is actively engaged in combat with the hired guards. The couriers are actually either shackled or just standing near their burning carts, which is shown at that particular timestamp.
Nisha’s dialogue is particularly interesting, especially the line, “The only thing these were guilty of was accepting a good-paying job to transport supplies.” Combine this with dialogue from some Hired Caravan Guardians, “I didn’t sign up for this!” and “This was supposed to be an easy job!” I almost wonder if the vulpera were fully informed of the conflict, or if they didn’t fully understand what they were getting into. Like, it doesn’t feel like they were aware they were transporting war supplies in an active war between the two superpowers.
We’re not sure who started the actual fighting. Likely, the Alliance came up and said, “forfeit the supplies”, and the caravans said, “nope”, and the Alliance responded with, “flame hands we weren’t asking”. Given their characterization, I don’t think the Alliance would have tried to burn the caravans with the vulpera in them, but would likely try to forcibly remove the vulpera from the caravans. This would have prompted the guards to act and attack, and the Alliance military would have the right to defend themselves from attacking guards, even if the guards weren’t a part of a formal military. But, the guards would have been in the right to defend their caravans.
The Alliance is there to fight the Horde, not for conquest or subjugation. Likely, they would have let the vulpera be, but certain vulpera aided the Horde, which puts those vulpera at odds with the Alliance’s objective. So, from an Alliance perspective, they’re just shutting down supply lines while trying to minimize collateral damage. However, the vulpera do have hired guards, and the guards are fighting the Alliance, so they have to defend themselves.
However, that doesn’t matter to the vulpera, or to Nisha. Regardless of whether or not they fully understood the scope of their involvement, they saw invaders attack their homes and burn the people trying to defend their homes. From a vulpera perspective, the Alliance are hostile invaders, regardless of the grander scope of things.
It does draw a lot of parallels to the War of Thorns. The attacked peoples aren’t necessarily aggressive, but they are aiding a hostile superpower. The attacked peoples see a foreign invader, and they’re just trying to defend their homes. Whereas the Horde was in the wrong for starting the War of Thorns, the Alliance was in the wrong for attacking the caravans.
For example, Nisha suggests the vulpera were just “accepting good paying jobs”. The Alliance may very well have been able to try to pay the vulpera to work for them instead. Basically, “If the Horde is paying you this much to help them, we’ll pay you that much and more to help us.” Instead, it seems like they directly became aggressive.