Via ABC News:

One of the FBI’s top investigators, tapped by special counsel Robert Mueller just weeks ago to help lead the probe of Russian meddling in last year’s presidential election, has left Mueller’s team, sources tell ABC News.

The recent departure of FBI veteran Peter Strzok is the first known hitch in a secretive probe that by all public accounts is charging full-steam ahead. Just last week, news surfaced that Mueller’s team had executed a search warrant at the Virginia home of Donald Trump’s former campaign manager, Paul Manafort. And the week before that ABC News confirmed Mueller is now using a federal grand jury in Washington, D.C., to collect documents and other evidence.
…
As chief of the FBI’s counterespionage section last year, he helped oversee the FBI’s investigation into Hillary Clinton’s use of a private email server, and he took part in the FBI interview of the Democratic presidential candidate.

Within weeks of the Clinton probe ending, Strzok found his office facing a new challenge: investigating Russia’s alleged efforts to influence last year’s presidential election, including a cyberattack on the Democratic National Committee.

What do we know from this?

We know that Strzok left under a cloud. He is a Senior Executive Service equivalent and you don’t move a former section chief to HR unless he’s about to get the heave-ho. We know he was hired on/about July 14, so he lasted a month. We know he was one of the two agents who interviewed Hillary Clinton.

We can surmise one of three things caused Strzok’s defenestration:

He was fired for personal misconduct. This could be any number of things from being caught up in the leaks of info surrounding the Hillary Clinton investigation or the early part of the FBI’s Russia probe or the leaks about Mueller’s investigation (think the week-after-the-fact leak about the raid on Manafort’s apartment) to moral turpitude (assuming that is an issue anymore) or some other problem.

He was fired because he wanted to go somewhere Mueller didn’t want to go.

He was fired because he didn’t want to go somewhere Mueller wanted to go.

All signs, at this point, indicate some subset of reason #1. If he’d been removed because of tactical disagreements with Mueller over the investigation, a soft landing, not in HR, would have been arranged for him unless the disagreements became mutinous. That is pretty unlikely given the profile of the guy involved.