Thixotropy

Newtonian fluids have viscosity that resists displacement proportional to the force applied.
Thixotropic have greater vsicosity for small forces but are fluid for large forces. (nd paint)
Bingham fluids are fluid for small forces and stiff for large forces. (potty putty and saturated custard powder or corn flour)

The second 2 are non-newtonian fluids.

The reason for the behaviour is at a molecular level and is associated with different scales of intermolecular forces within the fluid but I do not know the precise reason.