Is Electromagnetic radiation released, because of difference in
temperature? or because instability of electrons inside matter (atoms
and molecules) ?
It is because of temperature difference because temperature difference is a difference in the magnitude of the motions (kinetic energies) of the atoms and molecules of objects, and those motions reflect a difference in the magnitudes of charge- acceleration or dipole acceleration which produces electromagnetic radiation.
The main differences between heat transfer by radiation vs conduction and convection, are the mechanisms by which the kinetic energy of the higher temperature object is transferred to lower temperature object and the medium necessary for the transfer.
Heat is energy transfer between two objects (in solid, liquid or gaseous state) due solely to a temperature difference between those objects.
Although it is not a formal thermodynamic definition of temperature, an operational definition of temperature is that it is a measure of the average kinetic energy of the atoms and molecules of an object (the so called kinetic temperature). The object with higher average kinetic energy has a higher temperature than an object with lower average kinetic energy.
Heat transfers kinetic energy from the higher temperature object to the lower temperature object. This results in a decrease in the average kinetic energy of higher temperature object (a decrease in its temperature) and an increase in the average kinetic energy of the lower temperature object (an increase in its temperature).
The mechanisms of heat transfer (conduction, convection, and radiation) determine how this transfer of kinetic energy occurs.
Heat transfer by conduction involves direct contact between the objects so that the atoms and molecules of the higher temperature object collide with those of the lower temperature object.
Heat transfer by convection combines transfer by conduction with the transfer of energy by the movement of fluids in contact with solids.
Heat transfer by radiation requires no contact between the objects and can occur through a vacuum. Without getting into details, objects emit energy by electromagnetic radiation due to the vibrational and rotational motions of their molecules and atoms. These particle motions result in charge- acceleration or dipole acceleration which is what produces electromagnetic radiation. The higher the temperature of the object, the greater the radiant heat flux emitted. Energy of the electromagnetic waves from the higher temperature object is transferred to the lower temperature object.
Hope this helps.