Aztalan's relationship with Cahokia can best be described as a frontier outpost on the edge of the Middle Mississippian world. This is a bit of an exaggeration as we know trade goods were travelling much further than what is present day Southern Wisconsin; but Aztalan does seem to represent the edge of where we can see direct Middle Mississippian entrance into a local community. It's hard to know if this was a colony of some sort, a town that "bought in" on Middle Mississippian influence, or a subjugated community; but it is extremely clear that the inhabitants of Aztalan did not have the best relations with their neighbors. There is evidence that the palisade was not only destroyed once middle Mississippian power waned, but also that at numerous points during Aztalan's existence the palisade was damaged by someone. Now to be fair we don't know exactly why they built the palisade nor who may have damaged it, but it is clear that the residents of Aztalan built the palisades for a reason. The site itself also has a lot of architecture in mound construction that we strongly associate with Middle Mississippian mounds and also numerous artifacts that can be directly traced to Cahokia.

I personally think geography played a huge part in Aztalan's place in the region. We don't see nearly as much interaction between Middle Mississippian cultures in Eastern WI as we do Western WI; which makes sense with the Mississippi running through the Western part of the state. Most rivers in this region flow into the Mississippi, however in Eastern WI there is a small sliver that flow into the great lakes drainage basin. Aztalan sits extremely close to this imaginary dividing line, you only have to travel another 20 or so miles east before you start having rivers drain into Lake Michigan. There is often a storng link of cultural areas and drainage basins, which makes sense if you remember that rivers would have been the most efficient way to travel long distances in a short amount of time Middle Mississippian culture seems to spread via interactions facilitated by the Mississippi and it's many tributaries. Aztalan was a site of regional trade even prior to the rise of Cahokia.

I think it's a fair argument to say that trade connections between the two communities may have strengthen their bonds and caused Aztalan to drift towards Middle Mississippian influence and away from local cultures. Was Aztalan founded by Cahokia? Most likely not. Was it conquered by Cahokia? Eh, again there isn't much to show of a conquest by force. We know the Cahokia dominated trade and in doing so seems to also have influenced it's partners to adopt middle Mississippian traditions. Aztalan was an existing trade hub and, to use modern terms, found itself flushed with cash once Cahokia began controlling the region's trade. As Cahokia declined in influence, so too did the impact of Cahokia on Aztalan. This mirrors what we see across the region, Cahokia's decline leads to this exodus of people from Middle Missisppian communities to other 'local' communities. To make matters even more confusing, many of these individuals appear to bring with them cultural elements practices at Cahokia as they return to existing communities.

I digressed a bit, but I think the TLDR for this is that Aztalan seems to have been a trade center that already existed on the boundaries of many cultures, and that Aztalan was heavily impacted as Cahokia spread it's influence via trade. Trade works best when the trader can slip in and out of the cultures they work in, and this cultural shift may have been the final straw that alienated Aztalan in the eyes of other local communities.