Do they?
Under the assumption they do, I would say the reasoning for boomers would be the most common reasons anyone dislikes boomers
1. They run the world and are responsible for it
2. They got to grow up and build wealth in an economic golden age
3. They are the haves while younger generations are the have nots
This is kind of ignoring that gen x is also in lots of power too
Millenials probably hate zoomers because its common for older generations to ridicule the younger ones, and popular zoomer culture is extremely embarassing and irritating in many cases.