Botanists around the world had been stumped by a newly discovered species of fungus. It was extremely difficult to sequence the species without killing it, but all they understood about it was that the mycelium had some way of merging with nearby plant species in order to obtain nutrients. What was highly unusual about this was that unlike other fungi, the relationship that this merging formed with plants was symbiotic-- it could utilize living plant tissue while simultaneously not killing the host. This relationship did cause concerns, however. Namely that this species was newly discovered in November and no one knew what would happen with the fungus during pollen season.
Then Spring rolled around.
No one was sure how, but the effects were immediate and happened simultaneously to everyone the world over. Suddenly, all the mens' dicks shot into the ground and rooted themselves there while the women sank into the ground until their vaginas could open up inside the soil. Everyone's pants were torn open when this happened, and everyone inside of a building was not exempt-- their bodies simply sank through the floor until they found some soil. Men had their testicles suck back up into their bodies and move through their penises into the dirt, where their tested would unfurl and form a mycelia-like structure. For women, much of the same happened, but with their ovaries.
Humankind had simply been turned into anthropomorphic mushrooms by the mycelianated pollen. And not the kind of anthro mushrooms from kids games that have the giant caps for heads. They were just humans, but their reproductive systems (and eventually their nervous systems) became one with the earth.
There was panic from people who didn't know what had happened to their bodies. Chemical warfare was widely speculated, and some even hypothesized they had gone to hell. Once people's nervous system's had integrated into the mycelia network, they began to be able to communicate with each other through their mycelium and have information travel great distances, which helped inform people about what had happened. The integration of the nervous system did also mean that it was excruciatingly painful for people to remove themselves from the ground, and doing so would likely result in paralysis or death.
Once the situation became a little clearer, there was outrage. People were furious that the scientific community couldn't stop this from happening, and no circumstance seemed to be able to quell the outrage. There was also an uproar in the transgender community, with a vocal minority becoming enraged that the mycelia did not respect their identities and had gendered them so rigidly.