It was the enormous wave of spam that hit around mid-2018, which made it difficult to sustain forum conversations and sparked serious debate about an "exodus" elsewhere. Many of us were reluctant to do so but when another website (DART) came along that was designed to look very similar to DDO people made the change in droves. Debate.org had been on the decline well before this due to functionality slowly breaking down and waning interest among the core member base but I think everyone agrees the spam was the final nail in its coffin.
Seriously, in early 2018 DDO sustained a game of Survivor that had 9 participants, who collectively made up but a small fraction of site activity at that time. Immediately before the spam it had reasonably high forum traffic.