I started seeing avatars pop up with strange looking, brightly colored animal characters in them (they didn't strike me as ponies at the time, most of them were of rainbow dash) on websites like deviantart, and thought it was just another fad.
Then I started seeing images show up, whole pieces of fanart (the very first that I saw was of pinkie pie needing a cup of coffee) in galleries and sprays on TF2 with these characters, and started to wonder what they were about.
What finally got me to look into the whole thing was when I was on EVE and the topic on our mumble chat (in a room of like...dozens of people) was about that show, and a guy said it was the new my little pony.
I was dumbfounded when I heard "my little pony," being that I grew up knowing it was a toy for girls, so I had to check it out.
I watched some clips on youtube, some of which convinced me to try the show (namely the ones shown in this very thread). Although I wasn't exactly sold on the first couple of episodes, I was pleasantly surprised to see that, despite it being all sugary and brightly colored, I was more intrigued than I was wanting to cringe, I still had to skip the songs, though.
I don't know what it was, but something kept me watching episode after episode, until I finally decided that I was in love with it and watched as much as I could.
I didn't join the fandom till about a month later, and I've been a brony since (which has been about a year and a half by now).