In the 20th Century, music, clothing, political ideas, and other aspects of American culture seemed to change drastically from decade to decade. Was the rate of change abnormal compared to other centuries? If so, are there theories explaining what made the 20th century exceptional?

I can't speak for anything but clothing, and when it comes to clothing the rate of change in the twentieth century was not exceptional.

That is, it's abnormal compared to most of history, but in the late 18th century fashion started changing more drastically and faster, increasing in the mid-19th century. From a modern perspective, the difference between clothing in 1805 and 1820 is pretty slim, but from a future perspective the difference between 1990 and 2005 is going to look just as tiny.

If anything, I'd say that change slowed back down in the 20th century. (For women. Men's clothing changes slowed way down at about the same time that women's sped up, in the mid-19th century.) What really allowed for a lot of variation in women's clothing was the movement of the fashionable waist - high, low, or natural - and the ability to add or take away skirt supports. The difference between 1868, 1873, and 1878 is extremely dramatic for such a short period of time. The growing emphasis on comfortable clothes and the natural body meant that there were fewer options for changing fashions. Variations in pant leg width or lapel size are much more subtle than a bustle.