Another factor is that the DC Universe streaming service series Doom Patrol recently cast Joivan Wade as Cyborg.
Also, it sounds like Cyborg originally had a bigger part in Justice League that may have set up his solo outing.
The genesis of this rumor seems to have been less one specific thing and more a confluence of factors that people thought pointed to Cyborg and Ray Fisher’s exit.
For one thing, the Cyborg solo movie was initially scheduled for a 2020 release date, but given the various shakeups at DC and the fact that we haven’t heard much on that film in a while, some people presume that it isn’t going to happen.