Most AOEs are telegraphed before they begin, ngl. Even for new content, if you have a passing understanding of what boss movements mean you can typically dodge aoes before they begin. I.E. Boss moves right arm up, typically that means the right side is unsafe (Either in a circle with a safety cone, or in a rectangle.) They begin this animation before they begin casting, sometimes, too. If the boss suddenly moves upwards, that might mean that either under them is unsafe (jump down) or under them is safe (some form of circle aoe).

It's not an exact science, but certainly accounts for a lot of what you're probably seeing, OP.