No R Demand?

[The Arrowz 3]

My city is about 85 years old, and it steadily climbed to a population of about 500,000 before starting to level off. R$, R$$, and R$$$ were all in demand, as were all CS and CO. Then, when it finally got to 530,000, ALL residential demand plummeted, suddenly. R$$ actually hit rock bottom... And as the massive residential R$$ buildings became dilapidated to R$, population bloomed to 553,000. But even R$ is in the negative, which means eventually I'm going to have a huge loss in population...

It's really strange, like some sort of real estate bust just happened. It was all going so well too; but then this happened, and I've paused the game before everyone flees the city...

Some more information:

-There are neighbor connections.

-Commute times are reasonable.

-Schools/hospitals are satisfied.

-Pollution is reasonable.

Any idea what happened AND/OR how would you fix this? I'll post pictures if needed.