I was an OpenBSD and FreeBSD user at that time DragonFly was announced so I remember the original fork. Improving SMP (or SMP at all) was a thing a few of the BSDs were working on around this time as I believe processors were starting to add hyper-threading. I think that was the core rationale for the fork but Matt Dillon had a few other things he was trying. There have been a few similar forks like this where someone wanted to go their own way some with better success than others.