The 18 core i9-7980XE is widely thought[1] to be a direct response to the 16 core Threadripper announcement. It was announced late, with much less info. It has a expected release date months later than the rest of the i9 lineup, and is using higher core count silicon that Intel has previously only ever used in Xeons.

[1]http://www.anandtech.com/show/11464/intel-announces-skylakex...