Rent control makes it harder to make money offering an apartment for rent (or at least not as much as you can get by selling it out). So owners are incentivized to take housing off the rental market and sell it instead.
Sure, they try to make that harder too. But the owner can always kick tenants out to move in himself/herself. And so that's what's happening now. Owner kicks out tenants to occupy it. Then they later can sell it.
And they can even AirBNB it while "occupying" it.