The orcs were a already put into the setting with the first few books.
After Sigmar was revived by Dracothion(a powerful celestial entity that collects souls and lost magics from destroyed worlds) they began creating the city of Azyrheim that surrounds the core of the-world-that-was where all the lost souls clung to. After creating his new city, the lost souls were given flesh and the humans, dwarves, elves, orcs, gobbos and ogres of the old world were reborn.
Obviously, ogres and orcs are volatile allies at best so Sigmar sent them forth on such missions as culling the deadly and supernatural beasts found throughout the realms. Sigmar was able to strengthen such an alliance by rescuing Gorkamorka from a creature made of sentient amber that claimed dominion over the realm of beasts. (It's pretty obvious that Gorkamorka just kept smashing at something that couldn't be smashed until it engulfed him)
So, with Gorkamorka in his pantheon of gods of order, the Mortal realms were in a golden age.
Then chaos came back, the pantheon broke up due to Nagash and Tzeentch's schemes and, of course, Gorkamorka and his hordes got bored and rampaged throughout the realms.
The orcs and ogres settled in the beast realm while the goblins focused on the realm of life.
Hope that helps, I summed it up the best I could.
Also, as a interesting thing about that recent AoS tourney, besides the winner getting free models and the best painted force getting a free monster, the three runner-up armies got showcased in the web store.
https://www.games-workshop.com/en-CA/Warhammer?N=102267+4294965182&Nu=product.repositoryId&qty=12&sorting=rec&view=table&categoryId=cat440002a-flatPretty cool.