Marking
The issue of marks overriding other marks came up a while back.
I always imagined it as the diverse amount of communication, verbal and nonverbal, that goes on in an adventuring group when they battle creatures of unspeakable power.
Calling out, making eye contact, sudden head movements, even hand signals...all this tells your party where you're going and what you're doing so that everything is done efficiently and well-coordinated.
The idea of one mark being set, then removed later on by another can easily be explained through this system.