America’s incarceration rate is in decline

> After peaking at just more than 1.6 million Americans in 2009

> But a prison is a portrait of what happened five, 10, and 20 years ago.

Is this just a result of the dropping crime rates since the mid 90s, but on a 20ish year lag?

That's what the article goes on to describe, yes. Declining crime rates mean fewer new prisoners, but high recidivism rates plus long sentences means many old prisoners are still in prison. As those old prisoners die off or for whatever reason don't commit more crimes after release, the total population declines.

Mandatory minimum sentences can be 10, 15 or 20 years depending on the quantity of drug and other factors. Often just for possession. The US spent several decades filling our prisons with people using those sentences, and we still do, just not as aggressively.