Wish I knew the full politics of this, but I doubt I do.
Turkey has always been solicitous of the Turkmen outside Turkey, including in Iraq and Syria. Russia has been bombing them, which can’t be very popular in Turkey or with Erdogan.
And Iran and Turkey have been enemies since the Middle Ages. Assad is an Iranian pawn. Turkey shares a lot of border with Syria and can’t view the potential of Iran and Russia taking over the whole. There has never been any love lost between Russia and Turkey. One does remember that the Crimean Tatars are a Turkic people. Russia, of course, is allied with Armenia against the Turkic Azeris. Turkey has reason to be concerned about Russian/Iranian forces on their borders.
But I doubt Turkey is preparing to invade Syria now. Remember that Erdogan promised to fight ISIS on the ground if the U.S. would establish a “no fly zone” over the Turks’ heads? Could have been the real strategy was to keep Iran pushed away from Turkey. It might have happened then, but with Russia there, I expect the notion has been abandoned. If Erdogan was concerned about Assad’s air force, he would be far more concerned about Russia’s.