Specifically Auburn: I think apart from the obvious explanations (general trend, common app, etc), there might some other explanations:
Location: Schools like Auburn (and Clemson) are located close to booming Atlanta. Auburn is well connected to ATL Airport via shuttle service and it might even get an Amtrak station in the future according to recent news reports.
Campus: The campus is located in downtown Auburn and very walkable.
City: The City of Auburn is very safe, clean and growing rapidly (new downtown hotels, grocery stores, apt buildings, bars, dining options, etc).
Academic rankings & selectivity: Auburn ranks in the USNWR Top 100 which probably makes a difference when compared to other SEC schools in the region. Clemson is another example. Also, Auburn’s current acceptance rate is 45% (it was 70% just a few years ago). Further, Auburn is a STEM school which is quite relevant nowadays.
New campus improvements: There are some major campus improvements and construction projects (massive academic bldgs, residence halls, dining and food market halls, 5star hotel) u/c or already completed.
Famous alumns: Apple CEO Tim Cook, Secretary of Defense Lloyd Austin, Charles Barkley, Bo Jackson, etc are world famous leaders/aufhorities.
School spirit & athletics: Auburn co-owns the Iron Bowl which is a major brand, has a pretty good basketball program, mostly good baseball and is very competitive in Olympic sports (current golf national champions, Suni Lee & gymnastics, dominated swimming in the 2000s and partially 2010s, etc). Some unique traditions include the Eagle flight and Rolling the trees…
Student body: Auburn’s student body is relatively wealthy and only ranks behind Vanderbilt in the SEC. It would rank #6 (behind UVA) in the ACC. This is probably attractive to OOS applicants who can afford Auburn.
Overall, Auburn ‘feels’ more like an ACC school rather than an SEC school (athletics is important but so is academics).
Since many ACC schools are quite popular, it’s logical that Auburn is getting increasingly more popular.