Clothing was always, ever since man started wrapping himself in various animal fur, a statement of a certain kind. Fashion, as a style popularized and excepted by many in modern day, just deepened that statement's importance. Choosing one style or another gave people in all cultures a way of expressing themselves, and as there was always a fashion followed by majority of society, there was another, somewhere in the back of it, denying and rejecting it.
Alternative fashion styles are tightly close to music genres of what is also labeled as "alternative", such as rock and roll, punk, gothic, hardcore, and all varieties that rose up from those. Many social and cultural issues are brought up by associating a subculture with a type of music and a way of clothing, such as denying status quo of classes, governmental policies, distribution of power, and identity policy which majority popularized. Identity, in fact, is the most important aspect of alternative fashion and as an embodiment of it, it produced a variety of styles, to show the same variety of opinion, discontent, or rejecting the traditional values which are always transformed in shape but steady in structure in the mainstream fashion.
Reimagining one's own identity is a process that is followed by changes in personal appearance and usually approaching values of a subculture which offers to individual a better view of society. Those values are mirrored in music, clothes and accessories. Influenced by vintage and retro fashion, dress styles, forms of visual and performance art, personal attitudes towards concepts such as individuality, consumerism, social constructs on behavior, and considerations of availability and utilitarianism, rock and roll and punk had a very strong ground to stand on when social criticizm through clothing became unseparable part of the genres.
Since the 50s and 70s which were the turning point years, rock and roll and punk styles got multiplied by many, and brought us gothic, cyber gothic (and other varieties of gothic fashion which is still one of the strongest alternative movements in creation of new gender roles and identities and social boundaries), industrial (strongly influenced by military uniforms), Beatniks, Metal, Heavy Metal, Glam Metal (and many subgenres of metal music which has their own set of clothing styles), Hippie, Riot grrrl, Lolita, Mod, Ero kawaii (and many, many, many Japanese influenced styles with a strong influence on Western fashion, alternative and mainstream, such as Elegant Gothic Aristocrat), and many more. Individuality is an important part of alternative fashion, but there are always patterns (otherwise we would not call it a fashion), and even punk fashion, as an extreme form of rejecting social system through hairstyle, piercings, ripped clothes etc has a certain structure which can except anyone who wants to join it.