My guess would be that the ship the stormtroopers disembark from, as rumours have it, to burn a village on the desert world, is the same ship that Finn steals. Temporarily stranded and unable to mount immediate long-distance pursuit, his superior calls in backup.
This leads to a dogfight and the crashing of Finn's fugitive vessel. Pursuit TIEs are unable to find him in the wreckage and so probots are dropped. The clip we see in the teaser trailer may have been preceded by footage of probots traversing the sand dunes. They leave the shot, and a few seconds later, Finn emerges from cover and then heads off in the opposite direction to the probots?