The guy was arrested by the FBI because there aren't any FAA police. The FBI took him for a medical evaluation before they charged him. It will take some time before we learn what the conclusion of that evaluation was and why the FBI decided to charge him.
None of us were there so we really don't know the nature of his outburst. Everyone assumes he is suffering from mental illness. Perhaps the FBI determined some other reason for his outburst, thus the arrest. None of us know.
Then, there is the issue if this is a mental illness did it just occur or did the captain know about it and was having it treated on the "QT" so he could avoid reporting on his FAA six month medical application? If that turns out to be the case that is a different felony, that would probably result in a new criminal filing and, if found guilty by a jury, a couple of years of prison time.
If, in fact, the outburst was the result of a previously unknown medical condition that would cause a sudden and severe psychotic outburst, the guy will likely not be brought to trial and, if so, surely not convicted. In fact, if that is the case the charges will probably be dropped. But, there is a lot of evaluation and investigation that first needs to take place. It is not an easy matter to determine whether he was being treated on the QT and thus committing fraud on his 6 month FAA medical application.