It’s due to the Gems created by “Born of…” traits forming a 4+ match before the start of your turn.
It’s essentially a resurfacing of this bug (free, extra turns due to either Potions of Explosion or ‘Omen’ traits):
- Inexplainable extra turn
- Enemy got mana from my omen trait and I got a free extra turn
- Extraturn on match 3 and skipping extraturns (I think the internal bug report may have been linked to this one)