My granite floor appears to be developing cracks beneath its surface. These cracks do not go all the way through the tile, but can be seen only in a bright light and only at certain angles. It almost looks as if they’re cracking from the bottom

The condition you are describing is called compression cracking. It is caused by movement of the substrate to which the granite is attached. If the granite tile is set over a plywood floor supported by joists, movement in the floor may be responsible. You should be concerned but not alarmed. Compression cracking may lead to cracking which would require replacement of the cracked tiles. I would keep a close eye on the cracks, and if they develop further, would recommend consulting a stone specialist.