We have to stop thinking in terms of Iran & Iraq etc and think of it as Shia / Sunni / tribe etc. Because post the removal of the evil Saddam and the Ba'ath party dictators that is what now exists.
So it isn't remotely surprising that the Iranian Shia will support their brethren in 'Iraq' and it is also not surprising that there is a unity evolving between the tribes to remove the foreign invaders.