The huge profits that could be made from the wine trade, both civilian and military, inevitably attracted corruption, so much so that during the early 1st century B.C. the Senate had to pass the Lex Cornelia de Repetundis which increased penalties on the ever-growing numbers of provincial governors convicted for such activities. See Crimen Vinarius.
The network of merchants (vinarii) who were responsible for the procurement and distribution of such large quantities of wine surely were caught up in these scandals as well. And the distrust of them spread to question marks about the quality of the wine that they were shipping hither and thither:
"So low has our commercial honesty sank that only the names of the vintages are sold, the wines being adulturated as soon as they are poured into the vats. Accordingly, strange though it may seem, the more common the wine is today, the freer it is from impurities." (Pliny, Natural History XXIII.34)