Most of the stress comes from expansion and contraction of the graphene when it's heating up and cooling back down again, which can be more or less pronounced depending on the substrate (glass, SiO, etc.). With chemical vapor deposition for example, often the substrate needs to be heated and the graphene has to be transferred from one substrate to another so there is still plenty of room for stress during the process. At this point any poor alignments of graphene sheets or individual atoms (I think there's three classifications for the edge of the sheet alone) can cause cracks.