Why does coconut milk separate when it is placed in the fridge?
Coconut milk is made of different components, like water, fat, minerals and protein. All those components have a different mass and when let sit in the fridge for some time, the components with higher mass (protein) tend to sink to the bottom as they are pulled by gravity and the components with lower mass float to the top (water, fat).

The same happens with regular cow milk, which is the reason cow milk is being homogenized to prevent the fat from floating to the top. The reason is that apparently people don't like it when fat floats on top of their milk.

See https://en.wikipedia.org/wiki/Homogenization_(chemistry) for reference:


  One of the oldest applications of homogenization is in milk
  processing. It is normally preceded by "standardization" (the mixing
  of several different milking herds and/or dairies to produce a more
  consistent raw milk prior to processing and to prevent, reduce and
  delay natural separation of cream from the rest of the emulsion). The
  fat in milk normally separates from the water and collects at the top.
  Homogenization breaks the fat into smaller sizes so it no longer
  separates, allowing the sale of non-separating milk at any fat
  specification.