Often it's political maneuvers up-top against a rival military commander. Mercian legions are no longer following the imperial "enlisting" model that would be familiar to us, it has become a warrior-caste system in which a soldier's sons will one day become part of the same legion, and they become loyal not to the king, but to a certain commander in particular (typically the one in control of lands where the family women remain, administering over crops and slaves). Mercia is in many ways ruled by warlords, with the crown having limited powers.