I would assume they are using the correct type of caulking but who knows? Perhaps your sink/counter is not secure and that causes the caulk to stretch up and down thus cracking. Just a guess :)

This is what I don't like about undermount sinks. If they are not mounted properly, they sink when you add weight to them, like fill them up with water. The sink needs to be mounted solidly, and the granite resting securely on the sink. Not just mount the sink up under the granite after the top is in place.

The sink was in first, then the granite. A plumber told us that the granite was not on perfectly even. He showed me with his leveler. When the men that installed it came out and I told them this, they used their level and said it was fine...

A top that isn't level would not be the cause. The caulking doesn't care about level. Unless the granite is rocking on something, it shouldn't matter. If it was rocking on something, the tear would be on the outer corners, but I can't see the whole sink to tell where it is. Try filling the sink up with water and see if you notice a difference in how it looks. Take before and after pictures from the exact same place.

The cracking is on the two front corners mainly. I think the cracking in the back was due to water. We didn't wait long enough to let it dry set before using the water.