From what I understand -

The key point where things didn't play out according to the Ascians' plan was when Cylva revealed herself to be "the Shadowkeeper" to Ardbert & co. The intent was to have him kill her, which would keep the balance tipped close enough to Light for a rejoining but not push it into Flood levels. However, against predictions Ardbert spared her and set his sights on the puppetmasters of the scheme - the Ascian Overlords Loghrif and Mitron - which manifested his own Crystal of Light. After felling the Ascians, there was no more anchor of Darkness on the First, which triggered the Flood.

I can't verify this because I haven't done the Healer / Ranged DPS quests necessary to unlock the final one (I'll get to it in a week or two by doubling via ACN), but this is my understanding of the situation.

I wish I still had a D&D group... playing vicariously through Critical Role is nice but disappointing...