I have many theories but few that I can remember at any given time, that being said a few things I've been thinking lately.
1. I think Shallan's mother was likely a very abusive person, maybe worse then Lin. I think that she was far more on the emotional and psychologically abusive and may have done things like forcing Wikim into the Ardentia despite his wishes to the contrary. This might help explain why she didn't seem to have any problems killing her daughter because of budding radiance.
2. other things involve the surge of illumination. I think that Illumination always has an element of Fortune or future-sight. This is obvious with what we've seen of Renarins powers, but we see some examples in Shallans sketches, the crew after the Wind's Pleasure sinks, the statue of Ash that's smashed, but also in the tower there is a representation of Odium and his void, the Thrill and maybe an other Unmade.
I also think that a bit of what Shallan can do could be considered similar enough to Allomantic Brass/Zinc in the way that people tend to be more malleable to what she wants when she's Lightweaving, this is shown when she convinces Gaz & Vatha to follow her and again in the meeting with the King, and of course Mraize didn't just kill her so that's nice. I feel maybe a Scadrian could notice this as they are more likely to be trained to understand when their emotions are being pushed/pulled.