All viscoelastic memory foam will do this. Even the very dense over 5lb tempurpedic. Polyurethane mixed with certain additives change the structure of the polyurethane and creates a foam that is temperature sensitive. The longer you lie on it, the more it retains the heat, the softer it gets. For example the Foam by Mail 5lb viscofoam has and ILD of 12 which is super soft, not firm at all.
It is critical you know what you are buying when it comes to synthetic foams as they can be formulated by density and firmness. You can read through this tread as another MUGster has similar questions Foams, density and firmness (ILD) but specifically geared toward latex, but the principle is the same.
Maverick