If your kava is legit, then it might be filtering and sediment. Even "legit" noble kavas will sometimes be ground incorrectly so that they're too fine, or have hidden fine particles. This has been the case with me, if I drink any sediment then I am slow and fatigued the next day, and sometimes it's unclear why because I don't have the normal runs from kava sediment or anything. Like Kapm said DHK kavas like borogoru will also have a negative effect on me the next day, but it shouldn't be too severe. When I first joined the forums all I did for the first month was basically ask why I was hung over all the time and try to diagnose the issue (which seems to be what you're interested in), however I rarely have the problem anymore.
I filter with a nylon, and my batch ends up being 8 cups of liquid in a pitcher with 6-8 tbsp of kava. After the pitcher sits in the fridge for a few hours, like 5-6 hours I guess, the sediment really separates at the bottom. When I drink my first cup I pour the remaining kava liquid into a second pitcher (or any container), keeping all the sediment out. I blast the 1st sediment-only-containing pitcher with cold water and let that sit until I finish the 2nd pitcher's kava. When I finish the 2nd pitcher there might be some grains of sediment at the very bottom, so I avoid that as well. Then repeat. This works the best for me because nylons are good at filtering, but if they were too good it would take forever to make kava and even then sediment will still get in the drink.
My worst recent hangover case was when I drank borogoru and didn't use this system, and got maybe half the sediment in my body. So even though it was a noble kava, the DHK and sediment put me in full-retard mode the next day. And I could tell also because I started getting dermopathy from that week of kava use.