Turns out a biter was stuck in one of the spawners. I think you can see it from the last picture I sent. I saw him and killed it, and it's back to normal now... wow. Just that one biter...

I didn't open your save but is there a fire still burning near the spawners? If it damages an enemy the enemy is gonna call for help and hunt you down!

I almost died checking, but the spawners are perfectly healthy and there isn't any fire present on or near the spawners.

EDIT :
I waited about 5 ...

Maximum spawners on map gen.
About 20h into my save.
I attacked a bunch of spawners with my flame thrower and some cluster grenades.
About 5-10 spawners keep sending spawned units to me.
My base quickly gets overwhelmed by these unusual attacks if I am inside my base.
I am very far out of their line ...