Behaviourism originated with the work of John B. Watson, the basis of his perspective on psychology was that it should not be concerned with the mind or consciousness.Watson believed that behaviour in humans, as in rats and primates, was the only thing that could be studied objectively. Watson based his work following on from Pavlov's work with animal response conditioning.
Skinner tested Watson's theories and elaborated on them. To Skinner several of Watson's ideas were incorrect, and he rejected the emphasis on reflexes and conditioning.
To Skinner, people only responded to their environement, and in that response they reinforced the consequences of that response.
The term 'radical' was mainly directed at Skinner although Watson and Pavlov also had the title applied to them. The difference between Skinner and other behaviourists was his direct and provocative approach, and his refusal to accept that man was different to animals, he disputed the idea that man had a mind and a consciousness.
Bear in mind that from an early age Skinner was a devout atheist, this may have influenced his insistence that man was nothing more than a machine that responds to conditioning. In essence, behaviour insists that we are biological machines and do not consciously act other than to stimuli.