I think the trigger for the '96 line coup' happened during the World Cup return

It seems to be a well-established fact that the team atmosphere was a mess even before then.
After Klinsmann was dismissed, the KFA ignored public opinion and
pushed through the appointment of Hong Myung-bo.
Looking back now, I suspect it was ultimately due to internal conflicts.
But because of Hong Myung-bo's appointment,
the public's perception of the national team turned sour.
Far from resolving the internal conflicts,
they were probably rotting even more from the inside due to the Hong Myung-bo issue.
Then, the interview boycott issue acted as a catalyst,
and I suspect the trigger for the coup was the return trip from the World Cup.
Hong Myung-bo, Hwang In-beom, Hwang Hee-chan, Kim Min-jae, and Lee Kang-in
returned together and were heavily criticized,
while Son Heung-min returned separately and received warm comfort and a welcome.
From Son Heung-min's perspective,
there was absolutely no reason to return with Hong Myung-bo and act as a scapegoat,
so he might have thought, "Why should I come back with them?"
But from the perspective of the '96 line, they might have felt
that their captain used them as human shields.
I don't think this suddenly exploded during a friendly match;
I suspect they had already made up their minds around the time of the World Cup return.