Three major reasons seem plausible so far:

1. The Grimm (especially those big elephant fuckers) took notice to the sudden spike in negativity following the attack
2. This shit is gonna cause a lot of issues between the nations in Remnant.
3. Dragging on the festival. To what end, I can't say, but I think it's because so many people from all over the world are visiting. If they want to do something big, now is definitely the time.

I think part of Cinders plan, is to create spite and chaos between the teams, especially team RWBY to attract the Grimm as you say.
When the teams are in dissarray when the Grimm finally invades, there will be no way to stop them. Nations fall, the whole world falls into chaos.

Personally I like this dark turn, the series has going. Of course its probably not gonna end in some crazy dark ending, where the whole world falls, and everybody dies, but i'm liking it so far. Someone had a theory, that this episode was Yangs fall into darkness. That she would turn against her friends, and join Cinder. That would be cool, but I don't really think that would happen.
Guess we will find out where this is going next episode. Can't fucking wait.

That's the one!
gj, I was far too lazy to find it