Is the failure of composite filling usually due to under jaw pressure or from tooth decay?

Composite failure: Composites or tooth colored fillings are chemically bonded to the tooth, failure of the filing may be due to a breakdown of the bonding. There are many things that may cause this including additional tooth decay.

Composite fillings: Fillings can fail due to fracture or recurrent decay. Periodic check up can keep everything under control.

Man made materials. : No man made material will last forever. All will break down by erosion and wear, begin to leak at the margin between the filling and the tooth and decay will begin or other situations. In your case my main thought is how long has the restoration been there ?If it has been 7-15 years, i would conclude that the restoration has probaly lasted its life expectancy.

Both: Can be an issue of overload or torqueing forces which exceed the bond and/or mechanical strength. Decay can also be a factor which provides loosening options for the filling. Often it is both.

Composite failure : The failure in the first 3-5 years is a restoration issue (e.g. technique or bonding material selection) and after that time period from secondary decay.