The Kurdistan Workers' party is a sworn enemy of Turkey. Turkey and Iran have don't a very good relationship and with the fight in Syria, it only got worse Kurdistan Workers' party is also supporting Bashar Assad's regime which is being supported by Iran. Pretty sure that explains why Iran is supporting them.
This is not the first time that Turkey has killed pro-Iranian militias. The dictatorship in Tehran is conveniently looking away every time we cross Iran's 'red line'.