A bombing just happened, which could explain this current round of blocks: http://edition.cnn.com/2016/11/04/asia/turkey-kurdish-leader...
I'm reading from this site: http://www.amerikaninsesi.com/a/internet-erisiminin-yavaslat... (sorry for the Turkish)
The official power comes from a statue which allows restricting communication when "made necessary for national security purposes or a situation arises which might seriously impact social order", this was the justification for blocking after the bombing at Ataturk airport. People sharing videos and images of attacks is not good for the public morale, and that's ostensibly such an important concern that it merits blocking social media.
This current round of blocks has suspicious timing and extent but as my sibling points out, it's a state of emergency, they don't need justification.
As for the justification, nowadays they don't really justify anything, they just do, and it doesn't matter if it is unlawful or not. Especially after the coup attempt, there's this state of emergency situation which allows the government to bypass the parliament which pretty much translates to no justification for anything, capability is enough and it is not difficult to find stuff to use as pretext. Many of the government's recent actions are gross violations of constitution and several other laws, all possible due to the emergency status.