How long have you got?Some of it is obvious, some of it subtle...Interessting...but if that is true (to some extent...what happened exactly? Why is UD now Top and HE Flop ?Were are the "big" changes in the certain army lists which made the difference? And HE were indeed on the Top in 8th . ed.?
T9a tiers have floated around quite a bit though, so your question doesn't even really make much sense, as it implies a fairly simple process from legacy to now. There are some marked differences even just to 12 months ago.the ranking now
The tiers are close enough that it would be quite easy to screw everything up, and it doesn't take much to flip things around quite a lot.
In 1.3 EoS were bottom. In 1.0 they were near the top. In 8th they were middle of the pack, maybe a bit above middle. Last September they were middle, now they are lower middle.
Some of it is meta.
Some of it is people being slow to adjust.
Some of it is just cumulative small changes in a system with tight tiers.
Some of it is sociological even.
Starting point was my (wild) guess (beside the "bad guys are stronger -...idea )that the current Ranking could be (at least in part....see DE which were clearly a Top Tier army in 8th. ed,) grounded on the former Army books of 8th. ed. , that is: if an army was strong /top tier there, it "started"with an "advantage" (reluctance to "weaken" them ..and the other way around) .
Of cause I don´t believe that it is that simple (see De and He or UD on the other side). But again, I find it interessting what might have caused that...and as such, @Palmu s answer was interesting to me).
Veteran of the Chaff Wars