My interpretation is that it’s mostly the idea that they believed that the incident was so important, it started to “bleed” into Ruby’s present in times of emotional distress.

If we want a stronger explanation than that, you could argue that that point in spacetime was probably weakened by the presence of the TARDIS and Sutekh at nearly full-strength, plus whatever weirdness was caused by the use of the time window to view those events, and the creation of the Memory TARDIS.