I'm typesetting a long document with a lot of text, and sometimes paragraphs are breaking across pages seemingly randomly like in the screenshot. I tried to see if it had something to do with my paragraph styles & keep options, but apparently it doesn't. So I'm really confused why its doing this.
It is easy to miss hidden characters when the text is justified. Have a look with the text set to left aligned.
Keep Options can cause this kind of behavior. If some paragraphs are set to keep all lines together, and are set to stay with the next or previous X lines, it can cause unexpected paragraph breaks like you have there. See image:
It can be hard to identify where keep options have been applied. To test, select all of the text in the story and disable all keeps options.