During this encounter, if you call out, attack, or otherwise annoy the walkers, you will have to fight them and the survivors will flee.  

If you sit and watch the walkers in this encounter, you will come across a lone survivor.  The only way to get anything out of the encounter is to approach and keep hidden until the last walker is pushed into the trap.  Afterwards, you can attack the lone survivor and the three survivors that come after (or fight them all at once if you feel tough).  They are equipped with a semi auto pistol, a pump shotgun, a carbine rifle, and a hunting rifle, so this encounter can quickly turn against you.

Once you kil them, you can loot all of their guns, plus a walkie talkie and a combat knife.

If you talk to the survivor(s) and put your weapons down, it will force you eventually to fight, or flee--if you flee, you will get hurt as they shoot at you.