The context from the previous page is that Kagari is talking about several years ago and starts listing things that Soushichi saved her from in the past. I was somewhat unsure at the time as well but with this chapter, I think it's pretty clear she's referring to this event.
Dug through since I vaguely remembered and found it, Chapter 21 mentions Kagari was almost kidnapped and Soushichi saved her https://mangadex.org/chapter/b86db47b-3f80-4674-96b5-191f15088de7/4
This is probably why Soushichi is overprotective of Kagari, and it's also probably why he tries to keep...