@riot why is autofill happening when I Q as fill

I consistently Q as fill to help prevent people on my team from being dumped into roles that they did not ask for.

About 80% of the time I'm support and the rest of the time I'm usually jungle. When I'm not dumped into support though I hear people in champ select groan about being auto filled into support.

Why does that happen? That doesn't seem like it makes sense in a game with over a million players on it and probably 20000 Qing in my range at the same time.