Divergent plate boundaries cause tensional stress. Tension is defined as stretching force, the kind of stress that results from pulling an object apart. Divergent plate boundaries pull away from each other in opposing directions, which causes tension on the ground in between the plates.
This tensional stress causes the block of ground in between the plates to sink and magma from the planet's interior to rise up and fill the resulting cracks. This forms new crust, which has tensional stress applied to it as well. The sinking block is called a graben, and volcanoes can form alongside it. The ground in between divergent plate boundaries is called a normal fault. The geographical feature formed by a fully stretched block of ground in a divergent fault is called a rift.