Why is Max Planck sometimes called a reluctant revolutionary?

Max Planck is often described as a "reluctant revolutionary" because he introduced the concept of energy quanta not out of a desire to overturn existing science, but because the force of his own logic left him no other choice.1

Planck was deeply committed to classical physical theory and held a cherished belief that the second law of thermodynamics was an absolute law of nature. However, in his efforts to derive a formula for blackbody radiation, he found that he had to abandon this belief and instead embrace Ludwig Boltzmann’s interpretation that the second law was statistical.1

Furthermore, to successfully derive his radiation law, he was forced to assume that energy is absorbed in discrete amounts rather than continuously. This discovery fundamentally conflicted with all established physical theory at the time, effectively launching a revolution in physics that Planck himself initially struggled to reconcile with classical mechanics.1