Territory, mates, and food.
What type of tetra and how many do you have? Depending on the species, they should be kept in schools of five or more like cardinals, neons, glo-lites, silver tips (silver tips are really nippy with each other), black neons, rummynoses, etc. Some however perfer to be in pairs or what I call loose schools. They don't hang in a tight school but rather keep a distance from each other and form pairs like bleeding hearts, black phantoms, red serpaes, even black skirts.