From what I recall, the initial halt to trade was a result of the Broken Front, where Horde troops attacked the Alliance while engaged with the Scourge, causing both the Horde and Alliance forces to fall in battle.
After that, it was the matter of the Night Elves skinned alive and hung from trees, which the Horde not only celebrated like psychopaths, but which Thrall refused to apologize or seek any sort of justice for, because the Horde would not have liked that.
Overall, it was a mess.