Why some Shi'i scholars consider Shaykh Ahmad to be an apostate?

According to Tonokābonī, the reasons for the declaration of takfīr were three: Aḥsāʾī’s views on maʿād, meʿrāǰ, and the nature of the Imams (Qeṣaṣ al-ʿolamāʾ, pp. 44-48). Although Baraḡānī seems not to have referred to it, the shaikh had already been involved in discussions on the nature of the divine knowledge in Isfahan as early as 1228/1813 (Šarḥ al-resālat al-ʿelmīya, tr. Nicolas, Essai sur le Chéïkhisme IV, p. iv). As the takfīr was taken up by other ʿolamāʾ, the charges came to include further points. Raštī mentions some of these in his Dalīl al-motaḥayyerīn: It was claimed that Aḥsāʾī had said all the ʿolamāʾ from Shaikh Mofīd to his own contemporaries were in error and that the Moǰtahedī (Oṣūlī) school was false; that he regarded ʿAlī as the Creator; that he held all Koranic phrases referring to God as really being references to ʿAlī; that he spoke of God as uninformed of particulars and maintained that He had two forms of knowledge, one created and one eternal; and that he did not believe the Imam Ḥosayn to have been killed (p. 40).

http://www.iranicaonline.org/articles/ahsai-shaikh-ahmad