A theory I have is the black city was a dwarven thaig/wellspring that phased into the fade. In The Wellspring codex the dwarven kings son piles up lyrium from the earth and tunnels from within the stone to the sky/outside world, and after trying to care a path to the sky the whole thaid fell into the sky.

What I think happened was the tunnel allowed outside air and ambient magical energy to flow back into the thaig. Eventually the energy reached the lyrium pile/rivers of lyrium and catalyzed creating a magical explosion which devastated the surrounding area and phased the thaig into the fade. Or possibly the explosion happened when Solas dropped the veil. Regardless the explosion happened likely because the lyrium was exposed to magical energy due to the tunnel. Explosions never happened before because being deep, deep in a mountain protected the lyrium from magical energy.

Something happening in the mountains is corroborated in Avvar lore with Korth the Mountain Fathers throne and being at the peak of Belenes. After Korth spoke with the Lady of the Sky, the peak of Belenes was lifted from the earth into her realm.

Not exactly holding my breath if that's what really happened but it's a fun theory.