What Myth said. Any stack without a leader can fall prey to corrupt and seditious tendencies. When there isn’t a leader around put the stack safely into a city.

Low loyalty commanders can be trouble too but they can be dealt with by improving their loyalty or by keeping them constantly subordinate in a stack with a better a man. They do have a useful cavalry contingent after all. In extreme cases you can always send the traitorous types on a forlorn hope where they can die with honor and dignity before they cause you any grief!