How did the contents of private text messages become public?
Being Anti-Trump was the reason they were chosen for the job.
Mueller lost control over his controlled leaks.
Their existence is public but not contents ... yet. Mueller did not out them they were most likely outed by a white hat. Their reassignment is not Meullers doing.
They haven't, and the FBI will do everything in its power to keep it that way. Because protecting the reputation of the Bureau is far more important than ensuring its integrity.
If you meant "how did they become known," it appears to be the result of an IG investigation into the FBI.