It’s the same story every time. One thorough and oft-quoted explanation is here: „Resolve the collision..” strange behavior - #2 by dspreadbury
Basically, the combination of dynamics, lyrics and low notes with slurs has made the systems much taller than expected on this page, throwing off the rough estimate which forms the start of the casting-off algorithm. Save from adjusting global settings like the rastral size, a manual frame break should suffice to fix it. Or maybe you can fiddle with the casting off so that these tall systems are spread across two pages instead? Two systems on a page would look a bit odd and empty.
@TonH this happens even without any fixed number of systems per page. The overfull % is a symptom, not a cause.