Basically, someone a while back made a thread on /mlp/ about Mando being a pedo. The thread itself wasn't anything special, it wasn't big or active by any means, and should have been just another random unassuming thread produced and then lost within the swirling vat of chaos that is 4chan.

However, some dumb fuck thought it'd be funny to get a screenshot and show Mando himself, who promptly freaked out as if his personal reputation was under attack (which, of course, is ridiculous. I hardly believe that anyone actually thought he was a pedo, it was just a stupid joke thread). So, he starts making a bunch of butthurt comments on twitter, and actually brings some of the show's staff into the mix by bringing this retarded thread into the limelight, and all of a sudden everyone is up in fucking arms.

Now, since 4chan is mostly full of trolls, they acted exactly the way you would expect. When Mando acted butthurt, they took the joke and multiplied it ten-fold. Mando kept responding to the trolls like a dumb-fuck, so the trolls kept going, and everything just snowballed from there.

Now Mando is eternally lambasted as a pedophile on /mlp/. Personally, I don't feel sorry for him one bit. He completely brought this upon himself, and he's shown in subsequent encounters that he hasn't learned his lesson about responding to obvious trolls.

@Zancrow
Oh, apparently MandoPony and the voice actress for Applebloom got to be close buddies, or something like that. Naturally, half the internet assumes they must be a sexually active couple.