Hello,
There is an auto disconnect feature. It is set up so that if there is no user input for a certain period of time (I think 10 min's but could be wrong) the session is disconnected.
I is done this way to avoid lag by having avatars performing automated functions (like walking/running forever while your not even there) disconnect.
In addition to the lag avoidance it is also a good safety net in case your avatar is bugged or something causes your session to terminate improperly. As many can attest - there have been cases of people not being able to log back on as there avatar is already logged on.
I do not know if this function can be disabled. If it can, I would advise against it.
Good luck,
Grounder