I/their superiors forced them for a few stops, and then realized I could do both! And the dialog where they talk about what they learned by being forced, and what they might have learned if they went on, vs. what they could have forestalled by going at once and what would certainly have happened without their escape -- is already writing itself.
And I realized that a character whom I had intended to kill off later would certainly refuse to come -- eh, it disposed of him -- and then that he would be forced to follow (partly for his own greed) and then die --
A switch that meant that I was introducing a new character to the party before the old character died. How un-RPGish. (And one might also note that while they have a few points in common, it would be an odd player who ran both of these.)
And, of course, when three characters run into a fourth in the dungeon, albeit close to the door. Given the reasonable level of suspicion -- this is going to be an interesting scene. Much of it will determine what the three learned of him first, retroactively -- enough to move the scene.