I have asked this question before, but didn't get a satisfactory answer. "When walking on the beach at the water's edge, why is some sand hard and compacted while other is soft and waterlogged? It is the same time, tide, wave action and wind"
It can depend on the structure of the beach. Sand is the remains of rocks which were eroded and transported and as you might expect, the hard and therefore durable bits are more likely to form he grains (quartz). This (arenaceous) material will be free draining and the sand will dry out quickly. A beach which contains more clayey (argillaceous) material will be less free draining and when waterlogged will not support people walking on it so well.
I remember seeing a demonstration years ago explaining quicksand (which is a more severe form of the soft watery sand you describe). The reason it behaved as it did was because water was flowing upwards through it. Could be an explanation.
Don't wish to spoil the drinks party but you cannot drown in quicksand, unless it holds you half buried till the incoming tide gets you before the RNLI arrives. We have hovercraft for this job.