Someone pointed out that the sole reason Ichika stayed at school to begin with was because of Fuutarou, or more precisely her feelings towards him.
Now that she has decided to quit, one could say that she wants to "pursue" her career. But we can argue that this is utter bullsh*t. She could try and finish high school, especially when graduation is only few months away, and this is much more reasonable. And what about her sisters? Does she not care about the bonding?
The true reason might be that she no longer likes Fuutarou (Sisters' War flashback), or that she gave up fighting over him - which explains why she joins the swing. And this is just simply scratching the surface of her character.