The Terman who would be king (or queen):
The first was already alluded to in the descriptions previously posted. Termen who show great promise are often thrown out of their home colonies by jealous Kings and Queens who are afraid of them usurping the nest. These are usually of the Archon class, but can be any class. Others may simply wish to start on the path to being able to rule a nest some day. In their society, the way this is done is to go out among other nests, and sometimes other races, to gain skills and followers in order to either challenge the existing royals of their original nest or another nest for leadership, or gather enough followers to start their own new nest. All of this requires wealth and fame.
The not-so-quick and the dead:
Termen respond vigorously to a breach in their colony's defenses. Termidons rush to the scene to block entry to invaders as other casts, led by Terrarch earth seal up spells that move earth and throw up wall of earth. Often the breach is sealed behind the defenders, who give their lives to protect their home and nestmates. This sacrifice is such a great honor, and the danger of attempting to rescue survivors outside the nest so great, that by their custom, all defenders caught outside are considered dead and mourned as heroes. A terman of any cast that manages to survive, but is caught outside is dead to his colony and may not return. They are simply called the "Lost" and wander in search of new purpose and to erase the shame of surviving as others fell. From this unfortunate beginning many a great terman has arisen. Cut off from their home societies, they are far more likely to interact with other races and mingle in strange society.
Many castes much range widely, though usually underground for rare reagents (alchemists), creatures to tame (Swarm Lords, Theros, Slavers), or just new and powerful weapons. The most often place to find them is travelling to dwarven cities, but their paths could lead anywhere.
This is the most common manner in which a terman finds itself outside of its nest. Seasonally, they must undergo mass foraging raids. These usually are aimed at the arborial cities of elves, but they will raid any promising source of well cured wood. Wood with the taint of magic is a plus, and many a staff and wand have disappeared in a terman warren. In the confusion of these raids, many can become separated. Although they are not considered as "Lost", they do suffer a stigma if they need to break back into their own home tunnel complex. For doing so may tip off invaders where entrances are hidden.