In the US this is sometimes called a schoolmarm (alt school-marm) there are a number of ways they can form. OSHA Logging terms
As mentioned in the existing answer some trees like willow grow very aggressively and a low cut stump can have several shoots that grow in to trees.
Two or more seeds growing in the same location can grow multiple trees. If there is sufficient sunlight neither new sapling will have a significant advantage and both will prosper. This can occur naturally or from manual over planting.
Sometimes two forks of a tree will grow about the same. This can be an internal event for the tree, or due to an external trauma like the top of the main trunk of the sapling being browsed by wild life, leaving lower branches to compete to become one or more primary trunk.
Schoolmarm's pose risks on two fronts.
- The junction between the trunks is more likely to catch water and/or debris, leading to rot that will cause it to fail (die and/or fall) earlier then it neighbors.
- It is very difficult to cut down the tree safely, without good access to back side of the tree it is difficult to control the direction of it's fall. Directional tree felling