Why do the Gamemakers what to chase the tributes from their hiding places in The Hunger Games?

The Hunger Games are not just playing out inside the arena; they are being broadcast to every District and are required viewing for citizens. The Gamemakers are controlling certain aspects of the game to keep it "entertaining" for everyone back home. When Katniss finds herself in a tree at the end of chapter 12, she sees a wall of fire bearing down on her. She is forced to flee, as is every other tribute in hiding. She realizes that there have been no deaths that day and likely not any fighting, either:

The audience in the Capitol will be getting bored, claiming that these Games are verging on dullness. This is the one thing the Games must not do.

She realizes that all the individual tributes (with the exception of the Careers, who have a tenuous alliance) are all probably spread far and wide in the arena and are most likely hiding just like Katniss. The fire, then, serves to drive everyone out of hiding and force them into closer proximity to create conflict that results in deaths.
And the Gamemakers' twisted plan is quite effective in doing just that.