Two possibilities i am aware of:
1, there is a random event "the spider queen"(introduced through a full-screen picture like the one you see when entering wildlands for the first time) that populates the world with tons of spiders. There also is a new lair containing the spider queen herself placed in the world.
2, there is a regular quest where you find a cave blocked by a boulder, when you decide to remove the boulder and go inside you are informed that 'millions of spiders rush out past you'. You get to fight a hoarder spider inside, but after the quest it done all those spiders that rushed past you now plague the world around the lair.