More or less agree with kuros here, humanity was an anthill. Blob/mycus/triffid have a multidimensional war going on, we happened to shove new land in between them all and they had a tiny skirmish and the blob accidentally wiped out practically all of humanity. Myucs/triffid just showed up and tried to grab some of the empty space. Meanwhile humanity is franticly trying to not die and find a way to start a new anthill that won’t get instantly destroyed.
Blob just naturally expanded into open space and squished some ants. Mycus saw unclaimed land and tossed some mindless terraformers through the portal. A few triffid hearts decided to get away into some fresh space and set up their own groves. None of them are really trying to kill everything, and most don’t even realize there is anything to kill.