First, it might not be sand. It could be pollen or very fine silt (if you've recently had rain). One way to test is by brushing. If it fluffs up and disappears it's not sand. OTOH, if it moves along with the brush or feels gritty, then it could very well be sand.
If it is sand, it could be a broken lateral in the sand filter. A common telltale sign of this would be piles of sand about a foot or so from a return. The sand is so heavy is makes it only a foot or so before falling and piling up on the floor.
Post a pic if you can - it would definitely help us diagnose.