Some people thought it was a hilarious amazing way to troll by pretending to like a little girl's show. Soon other people thought "hey I'll troll these guys by saying I like it too". Eventually there was a whole ton of people all trolling each other, all of whom think everyone else is full serious. Eventually they thought of the stupidest name possible, 'bronies', all of whom agreed it was a good idea - of course they were all trolling and in private having a good giggle at how dumb everyone else was.
That's how bronies happened.