My accepted paper now appears as "under review" - could it still be rejected?
It seems to me that the conference committee accepted your paper to be presented.
The related proceeding is then  subjected to its own revision according to the journal standards.

This is what I get from your Q - in which is not that clear who has accepted what - and it is a common work-flow for many conferences when the proceedings are to appear in a journal, albeit in a special issue of it.

Alternatively, the editor at the journal could have decided to consult one more referee. This can happen, of course.
But in this situation you shouldn't have received any acceptance letter from the journal, yet, but only that coming from the conference organisers and regarding your actual presentation at the conference only. Hope it helps and good luck.