Apparently the problem lays with Kava lactones which make up only 3-10% of the root but can be found in concentrations of 30-80% in other parts of the plant. Extracts using the whole plant contained high concentrations of kava lactones and people who either had a pre-existing weakness or who hit the anti-lottery got ill or died.
Dragull, K., Yoshida W.Y., Piperidine Alkaloids From Piper methystine, Phytochemistry, 63(2) 2003: 193-198
[5] Nerarkar, P.V., Dragull, K., In Vitro Toxicity of Kava Alkaloid, Piper methystine, in HepG2 cells as compared to Kavalactones, Toxicol Sci, 21, 2004