The game puts an AI when someone who is matched leaves. For example, Pantheon finds a game for 4 players. One player leaves (cancels) while it's establishing connections, and then he's replaced by AI.
The game does not randomly insert AIs anymore, there was a huge outcry about that in the beginning and they took it out.
If anything, people are avoiding Pantheon because 2v2s on Crucible is starting to wear a bit thin..