Since Sejuani is getting nerf'd

It's her ult that hurts the most.

Not the part where you miss it (if you miss it, they're just going to flash and get away anyway), but the part where you throw it in teamfights. Going from 30% from 90% is pretty hefty for a champion that was relatively balanced (if strong), especially since it wasn't even affected by cinderhulk. So they're not adjusting what made her too strong (the damage from the HP bonus), they're nerfing what wasn't changed (the utility).