The events of Volume 7 brought into the light the true form of the spirits as Wescott claimed, the Inverse Mode. In Tohka's case is was the negative emotions she saw as Shido was nearly killed, while for Origami it was the realization the spirit who killed her parents was herself all along.
But what about the other spirits, how do you think they would go into an inverse state? Here are the reasons I believe would make each Spirit go Inverse.
Yoshino - Yoshinon is her most important friend, and as we know losing Yoshinon causes Yoshino's powers to go out of control. Yoshinon at one point lost his head and got it stitched back on later.
So I believe for Yoshino to go Inverse, Yoshinon would have to be destroyed beyond repair while she watched it happen. Yoshino's powers may be unstable when Yoshinon gets separated from her, but if she saw Yoshinon "die" in front of her she would likely feel the same way Tohka did after seeing Shido nearly die.
Kurumi - Kurumi's ultimate goal is to kill the First Spirit and prevent the space quakes from ever happening, but we also barely know anything about the FIrst Spirit and the only one who does know anything about the First Spirit is the Second Spirit who is imprisoned by the DEM somewhere in the world. I believe for Kurumi to go Inverse she has to learn her ultimate goal has already happened... and been the cause of the first space quake.
If she were the find the 2nd Spirit and she learns from her that the 1st Spirit was already dead and that her caused the first space quake 30 years ago, she would realize all the deaths she caused were pointless as her reason for causing them was already accomplished. The fact everyone she killed would never return would cause her the cruel horror of her actions to finally sink into her and she would goes inverse. This is also how I believe Shido would finally seal her powers.
Kotori - I believe Kotori would go Inverse the same way as Tohka did, she simply didn't at the same time as Tohka because she didn't see the same horror as Tohka had due to being on the Ratatoskr. Had she been with him she would have likely gone Inverse as well.
The Yamai twins - They're a bit harder for me to figure out, but since the two care about the other so much if one was in a situation the other would die the emotions felt by one of them would make both of them go inverse since they were originally the same being.
Miku - I believe Miku would go Inverse if her music career met the same fatal crash it did when she was human, but it would take more than just that, I think the fear of Shido possibly not staying her fan like he promised would overtake her mind and cause her to go Inverse.
Natsumi - For Natsumi I have the hardest time trying to figure out since her true nature hasn't been showcased as much as her pre-sealed nature. But if I had to harbor any guess using what has been shown, its being put into a situation where all the stereotypes she had seen during her pre-sealed state and making it seem like people were taunting her with them. The horrible feeling she'd experience would cause he to go Inverse.
Those are my ideas on how the other spirits would go inverse, I'd like to know what are your ideas on how the spirits would go into their Inverse states.