While a completed traffic stop can evolve into a consensual encounter between a citizen and a trooper, it can only do so if a reasonable person in the same circumstances would feel free to decline to answer the trooper's questions and leave. In this case, however, because the trooper completed the traffic stop outside Guerrero's presence and because the released driver never returned to the minivan, a reasonable person in Guerrero's position would not have realized the traffic stop had ended and he was free to leave. Therefore, as far as Guerrero was concerned, the traffic stop did not evolve into a consensual encounter.
I suppose this is an example of the federal court also being quite stingy about these supposed transformations from traffic stops to voluntary encounters. (At least I hope so!).
Here is Fourth Amendment.com's coverage of this case.