From there, all Justice League’s theatrical cut explained to us was that Vic was in an accident crippled him and killed his mother.
With the Cyborg movie (if it does indeed happen), the plan appears to be to dive into the specifics of that incident and offer up a more robust emotional throughline on Vic’s path to acceptance of his new role as Cyborg.
The earliest canonical footage (a.k.a footage that made it to the big screen) that we have to work with for Cyborg is the Batman v Superman: Dawn of Justice flashback showing his transformation as he fused with the Mother Box.