It's pretty obvious that US/UK/Israeli are involved behind the scenes. It's a game of chess with Russia and Iran. At the very least, tacit approval was given to the rebels. I suspect a lot more than that also. The speed of the rebel advance is as suspicious as the speed of the Isis advance, when they swept through areas of middle east on a blitzkrieg to create a caliphate.
The Assad government was being propped up by Hezbollah and Russa. The former just having been battered by Israel and the latter being overstretched in Ukraine, it was a perfect time to strike. With three million Syrian refugees in Turkey as a result of Assad, Erdogan has been supplying them for a while and has simply had enough of Assad.
Mohammed al-Jolani has been involved with Al Qadeia and ISIS in the past and Hamas has congratulated him on his victory over Assad. What will happed to the American backed Kurds is anyone's guess.