You have one of the expressions which are difficult to manually execute.  The best explanation I can offer is that since it is a cubic term, 6x^3, three factors will be generated. 

The numeric term -8 will have 1, 2 and 4 as factors and either all three are negative terms or only one of them is negative in order to result in -8. 

Unfortunately, the best way to explore that particular factored result is to use the 'Expanded' tab and eliminate each term in order to examine the effect of factoring out any given term.