In the game modes where you can fight manually (e.g., Guild War, Campaign, Tower, Dungeon, etc.), the battle pauses for a short time when a hero or titan from the attacking team uses their ultimate ability. All of their allies and opponents are frozen until the skill animation ends.
In some scenarios, this may influence the interaction between heroes' or titans' abilities. For example: Attacking Nova stuns defending Eden during the animation of Burden of Creation: