Although I don't know if this is the case, here is my theory. Copper is highly reactive. Finished beer has significantly lower pH then wort (mid to low 4's vs mid to low 5's). It is possible that this could be the cause of the problem.
The Pilsner Urquel brewery used to be (and may still be) an all copper brewery which leads to considerable copper pick up. Some will also note that copper is used in fungicide, however I have never heard of brewers having problems with yeast health due to copper kettles, lauter tuns, ect.
Hence, it appears that copper pre ferment - good, post ferment - bad, and I would guess pH has something to do with it.