The Eight Phases are basically "limbs" of Morganna, would probably be the best way to think about it and since they can be considered as such, they are eight crucial components to "The World"'s smooth operation during Morganna's reign. People fall into comas because the nature of boundaries (Liminality) becomes distorted, the player's mind is more or less drawn into the game and is then prey to whatever Morganna wants done. This is partially unconfirmed but I think it works something like this, I'll make my argument using examples throughout the medias...
The Phase fields are mentioned in //Another Birth as having a hold on the person's mental state. BlackRose mentions that she felt a shudder run through her when Skeith struck her with his staff, this shows that the Phase fields are likely a place where the boundaries become less tangible. This is also noted in //SIGN, Mimiru and Bear specifically mention that Bitter Fantasy Mirror World feels "different" as if it is outside "some sort of boundary" clearly not a Phase field (since Mimiru and Bear both die there) but Mimiru did feel as if Bear's hand was slippery with sweat when he dived to save her from falling.
Again, while not blatantly said I think that Mai's power of being able to hear the "Bell" allowed her to avoid falling into the Phase field the second time around. (I'm still not sure how she awoke the first time, it may be she wasn't immersed enough to allow herself to fall into that trap) and I think we get a pretty good view of the player when they are trapped in a Phase field with the example of Tokuoka, who was clearly in battle with Skeith when he lost consciousness and his constant button presses implies that he was still fighting, rather than having lost.
Morganna only Data Drains people who get too close to the truth or can supply a purpose. It's hinted that the Bracelets inside the Guardians likely possess a prototype of Data Drain as they only cause collapses and foggy memory. Tsukasa was necessary as the copies were apparently not working to merge with Aura's psyche whether this was Data Drain or not is unknown to me. Sora betrayed her and allowed the party to get away, his Data Drain was different from the others though. (likely owing to it being the first one aside from possibly Harald) in that he was trapped in the game like Tsukasa, but he still had some measure of control (again, like Tsukasa) but Skeith could be activated at any moment.
Interestingly Sieg, Alf, and Carl all had dealings with Sora throughout //ZERO. It is possible however that Kazu had Really bad luck. Since Shugo's player was friends with the Hayami family when they were young he played on Kazu's character for a time and there he met the child Aura. It is possible that Shugo inadvertently got Kazu Data Drained because of his previous dealings with Aura.