Yesterday I learned that when the Archon King grabs your soldier as his last action on this turn, a Viper can then successfully bind the same soldier on aliens' turn (kinda pulling him out of AK's hands). With his next reaction (which happens after aliens' turn, when you move another soldier) he will slam the soldier he grabbed in the ground as if he is holding that soldier. Then the soldier proceeds to be binded by a Viper like 20 meters away from the AK.
This was super weird. The animation shows the soldier binded by a Viper basically around the corner, and the Archon King slams into the ground by himself (with empty hands), but this still does damage to the binded soldier.
https://www.reddit.com/r/XCOM2/comments/1g15vbj/archon_kindviper_doublegrab/