Quicksand can be thought of as a natural fluidised bed.
Water percolating up through the sand exerts a drag force on the sand grains.
If the flow rate is high enough, this upward drag force exceeds the downward force of gravity and effectively suspends the grains in the liquid flow. The sand particles therefore act as elements of the liquid phase and a body would sink through the mixture. If the flow rate increases further, the sand grains merely move further apart, so that the drag force always just balances the opposing force of gravity. There is no suction force created and quicksand is actually just a liquid with a unit weight around twice that of water. This is true, but it will probably be a dead body.