Who caused Rize to die? In the first episode, someone threw the crane in order to kill Rize, however that also prompted in having to save Kaneki from totally being devoured/killed . 

That was the point from where everything basically took off.

Was the fall of crane just a coincidence or was it deliberately caused by someone in an attempt to kill Rize and if so, who could have it been?
In the Manga near the very end Tokyo ghoul, Kaneki remembers the incident, and it was shown that 


   The Ghoul Group called the clowns did it as One of them, Souta, was there.


The specific incident summary on the Wiki confirms this. 

The reasons why are not clear yet in TG:RE, as there are several groups that could have been the reason why it happened, or it could have been spur of the moment.