You might find it useful to examine pop bands of the time and see how they impacted the fashion world. Consider the Beatles for example - the Beatles phenomenon was incredible in terms of its wide-ranging impact. What fashions or trends did such music groups help to propagate? Certainly #2 is correct in highlighting the impact of the use of recreational drugs in the "swinging sixties" and its resulting effect on fashion.
Certainly the drug culture affected fashion as the multicolored clothing and ornate patterns were due in part to the imagination and visions of those experimenting with LSD and other hallucinatory drugs. In addition, the Vietnam war influenced the styles of some of the hippies, who wore modified army shirts, etc. The burning of the bras, led by feminist Gloria Steinham, brought about loose-fitting blouses and tops for women. The revolt of the Baby Boomers from the conservative ways of their parents' generation effected the freer, more expressive styles and colors.
Another tremendous influence on fashions was the Sexual Revolution which led to freedom in clothing. Midriffs became much more visible than before as the hip hugger came into popularity along with the shorter top for girls. Of course, the bikini was introduced, too.