Synonyms for Unanticipated:

adj.

abrupt (adjective)
sudden, unexpected, unforeseen, surprising.
surprising (adjective)
astounding, surprising, startling, sudden, astonishing, unexpected, shocking, out of the blue, unforeseen.
unexpected (adjective)
unlooked-for.

adv

unpredicted (adverb)
out of the blue.

n.

unanticipated (noun)
out of the blue, unforeseen, unlooked-for, unexpected.

Other synonyms:

Other relevant words:
unlooked-for, out of the blue.