And since he managed to make himself Foreign Minister while being grossly under-qualified I am forced to assume that he is not that stupid (despite appearances). It seems far more likely that he felt compelled to do his part to torpedo the peace talks. Conveniently he seems to have forgotten that he represents the Israeli government and shouldn't be sending another message than the prime minister.
Shaffir to Herzog: Opposition failed to show leadership in political battles against PM (Haaretz)
from the article: Lieberman: This generation will not see Middle East peace