Soul Swap isn't widely used any more for affliction. It's a huge waste of shards. Most DoTs are applied individually. Can you post a log file so I can see what's going on please? Could be related to settings.
The general logic is that it'll cast all DoTs first, and once it's finished with that, it'll cast Drain Soul.
Okay, I'll get one tonight.
Basically, there's 8 people in the group fight and it's applying dots to one, then drain souling them.