There are enormous stacks of research studies but if we were to narrow it down to most likely*. 1st remember the problem was not with "traditional" or what we call aqueous kava beverage which is dry or fresh kava root/stump strained with tap water. The problem was pills and tinctures. Even then, at least, the FDA cases were questionable from the beginning with the way data was gathered.
*(3)– The Liver scare– nearly 20 years ago these pills and tinctures were linked to liver problems. We now know that this was due to acetonic extracts made with unsuitable raw products (peelings and/or unsuitable varieties or even different species) but that kava juice is very safe.