Why do the Mu'tazilah not trust the Companions and accuse them of lying?

The questioner's intent with "principles of the Companions" is unclear. The Mu'tazila were never known to disbelieve the Companions absolutely as one of their five principles. Rather, they relied on intellect, and because of it, they impugned the Companions who participated in the Battle of the Camel, as Wasil ibn Ata claimed that one of the two factions was sinful and rejected their testimony.