It might not be the only story, but it's very possible for growth in demand for housing quantity to outstrip growth in population. Say if there was a sudden move to working from home, which encourages home offices, concentrated in the class of workers with flexibility in their budget to spend more on rent (a class that's disproportionately large in NYC)?

It's also possible for supply of housing to reduce simultaneously. Say if there was a change in rental laws that strongly incentivized landlords to pull units from the market?[0]

[0] https://reason.com/2019/06/17/new-yorks-progressive-backed-r...