I've seen this too. titans do act strangely and sometimes they think they are in range when they aren't. I often take them out of fleets and micro them separately because of this.
FYI, a Sins fleet is a group of ships that you have grouped together and set one as the leader, so when you click on the fleet icon above the leader ship, all other ships in the fleet are selected. Not everyone uses this, but it can be convenient.
Engagement range is a set of behaviors where ships will attack any enemies in the gravity well, enemies nearby, or not auto-attack.
Both the fleet and engagement settings are in the same area as special abilities and strike craft construction. It might be worth checking to make sure you didn't accidentally set the engagement range to "local area" or "hold position".
