Your forests? Spiders do not spawn in occupied territory, and they do not stay in cultural borders. They obtain the cautious promotion until they have left, and do not attack, they simply try to flee.
SO what really happened, was you sent a worker outside of your borders to work, and it got eaten by a baby spider, wihtotu checking with a scout to see if it was safe first. Also, unlike vanilla FFH, spiders don;t just stay in the same place, they move about, and they specifically like to gravitate towards weak targets.. It's wild mana.... and there are many spiders, and baby spiders are great to catch and raise into giant spiders by animal handling promoted scouts.