Another one could be the death of her father. It is stated in canon that Hiyoko’s father is someone she truly cares about. It is also confirmed in SDR2 that while the Remnants of Despair were brainwashed by Junko they did the most heinous of crimes to others and themselves. It wouldn’t be too much of a stretch to say that Hiyoko’s dad was injured/hurt in the early stages of despairing manipulation, and then offed as a way to drag Hiyo under. 

All of Hiyoko’s dreams and thoughts of grandeur are built upon materialism. If everything falls apart, then so do those meticulously crafted fantasies. This part is seen as pure psychological hypothesis, but those who are intensely bullied usually have many forms of coping and defense mechanisms in order to calm themselves, or assure themselves that they are not as disposable as the aggressors try to convince them they are. One coping mechanism, for instance, is her air of haughtiness through childlike, better-than-though rudeness, and a defense mechanism of hers is lashing out at people with the rudeness. She sets herself on a pedestal, comparing herself to others through the act of dehumanising them, making the way she acts seem more realistic and just better than her peers. What if she is dehumanised too, as nothing but a pawn of despair? All of those mechanisms are gone, and that can really mess up a person. 

What people need to realise is that Hiyo can dish it out but she can't receive any of it back. She's sensitive, and has shrouded herself so deeply in these acts of arrogance and cruelty that she has become them. Haven't you ever met one of those people dripping with sweetness that are taken advantage of and are turned into stone-cold beings after? It's a defense mechanism. It's not right, but understandable and human. Hiyo is as susceptible to despair as, say, Peko is. It may not be the easiest, however with the right buttons pushed things can be triggered in going downhill.  

I have sooooooooooooooo many more oh my goshhh if you want to talk about them at any time please send me another message and I will continue to go to on more spiels, okay??