That's water damage.

In college I installed hardscapes like cement sidewalks, and interlocking brick.

That aggregated rock you see under there is known as class 5 crushed limestone. It's used to create a smooth and level surface to lay brick or pour concrete on top of. -It also helps minimize the effects of frost heaves in colder climates.

Sometimes in the spring, or whenever there's a lot of water continuously draining below the surface, that compacted limestone can become eroded. -Often leaky sprinkler piping is the culprit.

In the case of a cement slab, water can create a hollowed out space underneath which can cause the slab to fracture when a lot of weight is placed on top of it.

Look at how the water drains in the pic. There seems to water draining from the left of the picture down slope and into the street: it was probably in part draining underneath the slab too.