1) They have a lot of free time, they are mostly lonely people.
2) They often ingratiate themselves to a person or two on the group and use them to stay in the group. They may protest with these “friends” that their right to free speech is being curtailed.
3) They sometimes use “socketpuppets“, i.e. fake identities that may be used to sustain, or to inflame the troll’s position or theory or attack. At times the socket puppets’ names are anagrams or similar to the troll name. Thus a troll may engage in artificial conversations with himself. However impersonating multiple people is frowned upon by the more able trolls and is considered the lowest of the possible troll tactics.