According to Reddit user u/kamatacci, Vader fell backwards as Antonio Inoki was coming out to deliver a speech. While laying on the mat, Vader was given water by Keiji Mutoh and Akira Maeda to try and help cool him down. Vader then apparently rolled out of the ring and stumbled his way to the back without help.
Due to Vader having recently revealed that he was given a death sentence by doctors over a heart condition, many understandably began to speculate that one must have had something to do with the other. Vader attempted to allay those concerns with the following tweet:
The main event match@ Koreguen Hall Tokyo in a spot with Fujinamii Iwas dropped on my head which knocked me out giving me a concussion
— Big Van Vader (@itsvadertime) April 21, 2017
So the way Vader tells it, the loss of balance was from a bad bump he took during his match and not because of an issue with his ticker.