Without even looking at the link, my first thought would be rough preparation in foundational math courses during COVID virtual learning. Fall 2021 and 2022 were high school upperclassmen during that time, whereas fall 2023 freshmen would have been more at the foundational levels of alg 1/geom/alg 2.
I would also look at any changes to the structure of the math track, if there were any (I don’t know, just speculating), that would have affected these class years, maybe back in middle school.
If there is other standardized testing during middle and high school, such as state tests, I’d wonder what they showed for the fall 2023 freshman class year relative to earlier ones.