The reason of grooving on the triple junction formed by the two free surfacess and the grain boundary is the thermodynamic requirement of minimum entropy production. A bamboo like interconnect structure contains many triple junctions of this type as illustrated in below figure:

The rapid establishment of the equilibrium angle by atomic migration in the infinitesimal region of intersection produces sharp ridges that border each side of the boundary. These ridges tend to flatten by preferential mass transport mechanism. This flattening perpetually upsets the equilibrium angle and forces the groove to deepen.