The Heralds arrival signals a desolation. The period that the Heralds leave from Roshar following a Desolation results in a time of relative peace. When they are gone they apparently pay some kind of penance on Braize (Hell), are tortured. The time of the penance seems somewhat proportional to how many go to Braize, the more who go the less time they have to suffer while there. As if the penance were split amongst them. When their Penance is done they are allowed back on Roshar but again their presence signals yet another Desolation, and the cycle repeats. Thus when all the Heralds went to Braize the period between Desolations was shorter. Now that only Taln went to Braize this last time, he had to bear the ENTIRE penance of the Heralds which took A LOT longer than previously because only one man bore the brunt of that penance. As a result the peace on Roshar has been much longer than normal and some thought permanent. This is not so though as we know Taln has finally been freed of his Penance and as soon as he returns to Roshar a massive Desolation begins to percolate. Because of this massive penance he bore alone he has turned mad, his mind broken. Now, if the KR learned of this decision of the Heralds other than Taln to refrain from returning to Braize then that might be considered a betrayal, "why follow dishonorable leaders that would sacrifice one of their own out of perceived weakness". To top that off if they learned of the Oathpact and wondered why Honor would agree to such a thing could be considered another betrayal.
Just some ponderings on possible motivations.