Autofilled Into Another Person's Primary Pick

I picked top/jungle (1st/2nd) was autofilled into support, someone had support picked as their primary. The person who had support picked as their primary got autofilled into bot. Likewise, someone autofilled into top, they had bot as their primary. So bot swaps me for supp, I swap top. We become happy.

As a serious question; how the hell does that happen? I am flustered by the lack of thought that was actually put into autofill.