Aback is a mostly archaic adverb originally meaning at or on the back. So when someone was taken aback they were caught off guard by something coming from behind. From this derives the meaning of the modern idiom, take aback, usually inflected taken aback: surprised or disconcerted.
There are Britons in Berlin who get taken aback by the directness of Germans. [BBC News]
We’re often taken aback when a respected governor, political candidate, husband or wife is caught cheating. [NPR]
It was recognizable as self-replicating spam, but still, I was terribly taken aback in that first moment. [Edmonton Journal]