I thought I saw on Oink's signup a toggle that let you opt-in to auto-following other users. I was surprised to see that as usually it's opt-out (if at all). What this seems like to me is a glitch on Oink's end - not following their own preference options they expose to their users.

edit: I definitely didn't mark down to auto follow my friends. I just got the pig sound notification and it scared me half to death. Oooh boy my heart is racing.