My best guess, without seeing the plant, would be that contact would cause the damaged edges of the foliage. If the plant is in a high traffic area where it is being brushed up against frequently by people, animals, etc. the contact can cause rips and tears at the edges of the leaves.
There is also the possibility that an insect of some kind has found its way to the plant in question. It would be hard to say what kind of insect but if the damage is significant, it would likely be something large enough to spot at some point, like a grasshopper or caterpillar.