Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies
Apparently states had enemies and treason is defined as adhering to their enemies so if the state of Arizona made Mexico an enemy and California aided Mexico that would be treason under this definition. Its a good thing they put this in their because it keeps states from conspiring with each other to backstab other states.
First, let's get the full quote from the Constitution:
Section 3. Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort.
States did indeed have enemies. It is documented that there were a number of enemies of the states that were trying to kill them by shooting electrical currents and rays through the walls which surrounded the respective states. People would toss food over the walls in the hope of appeasing the enemies and making them stop shooting the rays through the walls. This action was misunderstood, and the people who were putting the food over the wall were put on trial for treason ("giving them (the enemies) Aid and Comfort").
They were never convicted, however, because the food they were putting over the walls, it turns out, consisted mainly of sweet rolls and coffee from Starbucks and, as everyone knows, in spite of what they call it, Starbucks coffee tastes like mud mixed with pieces of chopped up asphalt.
So the defendants went free and eveyone lived happily ever after. The enemies decided it wasn't worth it and gave up. The states tore down their walls. The enemies entered the states (legally) and began working at hotels and golf courses.