Who caused Rize to die?
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.