The second side trek presented in Dungeon #155 is yet another ambush. As much as this is repetitive and cheap, it's probably better than just spawning monsters on the far side of the map and have the PCs spend a turn running at them. Ambushes are a dynamic way to start encounters in an up-close-and-personal fashion, particularly in the context of otherwise quite open outdoor spaces.
It's still lazy, though. It opts out of the entire milieu of ways that an encounter can start other than combat.
The Slaver Encounter takes place around the same point in the adventure as the Wagon Ambush - that is, after the defeat of Irontooth but before players get too far into the keep. Here, the players are going up against a group of Bloodreaver hobgoblins from Thunderspire Mountain.
The set-up is obviously intended to strengthen the weak links between Keep on the Shadowfell and its sequel, Thunderspire Labyrinth. These hobgoblin slavers are in the area to negotiate with the kobolds, and when the players (accidentally) discover them in the wilderness, the slavers will be trying to defend their current slaves while also fighting to subdue and capture the players.
It doesn't work for its intended purpose. The hobgoblins are, necessarily, at an appropriate challenge rating for second level adventurers, and are therefore selling themselves short. Showing the players easily killable hobgoblins now diminishes their impact later in this very adventure, and significantly cuts the anticipation of their forthcoming appearance in Thunderpsire Labyrinth.
There's a secondary problem, which is that the players might get the wrong message out of this fight. An unwary DM can suddenly find that his players want to go haring off to Thunderspire to finish off these slavers as a sidequest on the way to the keep; it'll take some careful doing to steer them away from the forthcoming level 4 to 7 zone without making them feel railroaded. Better to avoid the temptation completely.