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.

This occurred to me, too, a month or so ago.  The problem is that the random events screens can show up and vanish in just a few seconds, leaving you wondering what the hell just happened.  That particular one is pretty damn nasty, because the spiders proliferate like mad.  Depending on what sovereign you're playing and how you play, it can be Game Over.  I'd suggest loading a save from before that point.

Perhaps this would be a good time to ask the Devs to post a revised list of the current Hot Keys, especially the control keys.

ctrl+U is a cheat code... you have to be running cheats to use it.

ctrl+R cheat completes all research.

ctrl+B cheat completes all buildings.

ctrl+T cheat teleports currently selected unit to location highlighted.

...there rest of the cheat codes are in another thread.