Why is Orianna getting nerfed?

Just curious on this, they're pulling 5 mr from her kit. She doesn't seem dominant or even that present in any type of queue, so... why?

I mean, I understand the Lee sin and yasuo buffs, but still... (just had to give those a nod since I know the first comment is probably going to be like "For the same reason that riot is buffing yasuo and lee sin: because balance team is stupid".