As an example, why does glass tend to break by means of intergranular fracture?
My analysis so far led me to believe that the tendency can be related to the intermolecular bond strength in a given crystal grain and the intergranular bond strength. A stronger bond means that better load bearing. If the intermolecular bond is stronger than the intergranular bond, then a material is more likely to experience intergranular fracture. But is this correct?
Glassy microstructure contains no grains, so its fracture mechanics is not "intergranular".
In a polycrystalline solid (which glass is not) the boundaries at which individual crystallites meet are discontinuous with respect to crystal lattice orientation. This mismatch means the crystallites are generally less tightly bonded together than the atoms comprising the interior of a grain, and such a material will usually fracture along the grain boundaries, as you guessed.
The question of whether a given material will fail in intergranular fracture or transgranular fracture is far more complicated a question that has to do with things like grain size, differences in chemistry between the interior and exterior of a grain, the degree to which the material strain-hardens on deformation, and so on.
You might try posting this question on the Engineering SE or try poking around a bit on wikipedia. The answer will be long and involved.
