You have to ask your question on Reddit, and you need to have a Reddit username. They usually start the AMA thread right before it starts. the celebrities have their own username, and they choose which questions to answer.
Usually it's done via phone, so the celebs aren't actually typing, but someone from Reddit asks them the questions and they answer on the phone and they type it down.
So usually this means they don't ask them any awkward or malicious questions.
Well, I guess the AMA went about as well as expected. Seagal's answers were short but full of some golden Seagalisms.
The Redditors took the piss and pretty much brought up every negative thing about him and made fun of him, including the rape allegations, sex slave allegations, him claiming he taught Anderson Silva the fron kick in UFC, him driving that tank through a guy's house killing his dog etc. It's endless.
I doubt he'll do anything like this ever again, since he's not a well liked person at all, so the serious questions will be far and few between.
EDIT: Oh my God I am reading through the comments and I am laughing my ass off.
Not surprisingly there are a number of people who claim to have worked on movies with Seagal and their stories are all negative. One guy even says he went to the same Jr. High School that Seagal went and according to a teacher Seagal stole lunch money from a kid in a wheelchair.