Mogred doesn't drink it though, but one of his henchmen, and even though the poison is not enough to actually kill, people will quickly decide it was obviously meant for Mogred and with the intent to kill. Why else poison anyone, right?
So now the moot is in danger of falling apart because mutual suspicions, which is exactly what the poisoner intended. Since the Companions are outsiders, Radagast asks them to investigate and find out the poisoner before tomorrow and the fateful vote.
Who could have violated the sanctity of the Moot? Was is Ceawin in fear of Mogred whom he sees as an obstacle of joining Woodmen? Was it one of the wine-merhcants from Dorvinion, who followed Ceawin to the moot? It was their wine after all! Maybe it was Ingomers other son, who is afraid of losing his heritage and jealous of the able war-chief Mogred? Or it could be a cunning plan by Mogred himself to destabilize the Wood-men community and make him seem the victim of sinister plot? And it could have been Edda who knows all sorts of Poisons, you know!
And what about those erring-men minstrels, they're certainly a suspicious lot? Etc.(it could also be a past victim of Mogred, finally trying to get some payback?)