I think it’s a combination of both. New York City is approaching herd immunity.
Ironic that nominally “locked down” NYC shows the quick spike with rapid falloff that you’d expect with a more open approach. Of course this isn’t surprising since NYC never closed their public transit — the ideal virus spreader — and thus were never really locked down. The result is they’ve more quickly approached herd immunity and are probably well positioned to avoid a “second wave”.