The big incident is very simple and very straightforward. I don't understand why some people have to make it more complicated than it is. Reread chapter 1078.
The marines were in their way to Egghead to face and assassinate Vegapunk, who is resisting. So they sent Kizaru, Saturn and some marines.
However, they were not expecting to see Luffy on their way to Egghead.
Now, this chain of events is going to culminate tomorrow and the outcome will shock the world.
The culmination leads to Kizaru facing Luffy and Kizaru losing against Luffy because initially, the plan was to simply stop Vegapunk.
Instead, the marines/WG look like clowns because one of their admirals loses against a yonko Luffy and Vegapunk escapes with Luffy.
This will shock the world and potentially trigger the final war.