Arenas from 'Hatchling Encounter' and above are affected by Arena Seasons. These can last from two to four weeks, and at the end of the Season, there is a partial Trophy reset.
Each Season comes with new Arena rules and rewards, such as boosting different elements during battles.
The dragon Orbs found in the Warrior's Chests vary from Season to Season and the higher you progress, the better the rewards within these Chests.
Remember that the Master of Masters Arenas also come with end-of-Season rewards. These are accumulative, meaning you will receive all the rewards for your tier as well as those below you.