Monsters on a quest are spawned through placed triggers on a ground called "encounters." The number and difficulty of monsters are balanced according to the estimated strength and power of the party. This is determined by by all the party members that are in the area/zone at the time that the encounter is triggered.
Problems can thus occur when you send a scout through a transition and far into the next area. The scout might trigger a bunch of encounters, and the monsters will be balanced towards a single character, leaving the party with very few monsters to fight.
When done deliberately, this is considered an exploit. However, I realize most people are simply unaware of the mechanics behind this.
Most quests have the encounters sprinkled throughout the entire area, but depending on the design of the area having the first few adjusted for a single scout can sometimes radically alter the nature of the quest.
As such, when scouting, refrain from going "too far" ahead of your group after a transition. Certainly you're encouraged to confirm that the transition itself is safe, and go ahead a little, but generally it's best to scout the transition, tell everyone else to follow you in, and then (when the party is in the area) to continue scouting the rest of the map.
Note that there's no issues with scouting past transitions that just lead to a different part of the map, and not to an entirely different area.