Roughly translated from http://www.golem.de/news/community-fork-und-neues-unternehme... :

Jos Poortvliet, Owncloud's former community manager and now responsible for communication at Nextcloud calls it a "new start". Karlitschek and Poortvliet give as reason for the new founding "structural problems" and some "economic decisions" of the Owncloud company, that caused great dissatisfaction. They do not want to give further details. Nextcloud's business model should be focused on long-term viability and sustainability. Apparently the Owncloud company could not offer that in a satisfactory measure.