After every rainstorm here in south florida, the landscape has been full of sand and gravel that I rake every week, but it continues to appear again and again out of nowhere - Anyone with a suggestion or explanation for this?
I can agree with Black Thumb. I have the same problem. But there is another explanation for it, here it goes. In any place where it is cold enough (like winter) to freeze the ground you will experience this. The reason for them to come to the surface is because they are better conductors of heat than the soil. So the stone will conduct heat away from the soil beneath it. The colder soil under the rock then freezes. And since when water freezes it expands, pushing the rock up. When the ground thaws a space is left under the rock, and the dirt around it will fill in the gap leaving the stone rest higher. And with this repeating it keeps pushing the rock higher and higher, till it reaches the surface. Of coarse since your in Florida this more than likely does not aply to you, but this is what I found!
Freeze-thaw isn't doing it in South Florida (nor "after every rainstorm"), and in normal weather conditions air transport is limited to dust.
So, your most likely option is critters that live in holes in the ground, from little ants on up through land crabs and larger things. The rain upsets their tunnels, so they clean them out.
If you remember your bible stories, you're dealing with the same thing that caused frogs to rain. This works through the evaporation of water lifting sand into the air, because it's so small, and light it gets lifted very easily.
I remember it from a show on the weather channel, but here's the wikipedia answer for you