Notice the awkwardness of the sentence below:
She was taken suddenly aback.
The problem is that this sentence violates the following rule:
When an adverb qualifies [modifies] a verb phrase, the natural place for the adverb is between the auxiliary verb and the principal verb. Chicago Manual of Style, Sec. 5.160.
In the sample sentence, “suddenly” is the adverb, “was taken aback” is the verb phrase, and “was” is the auxiliary verb; it helps the principal verb “taken” form its voice, tense, and mood.
When corrected, the sentence reads like this:
She was suddenly taken aback.