It has been discussed ad nauseum in theses forums. There are multiple reasons for the low scores.
1. Some players are that good. You are probably looking at "unlimited play" tourneys. Gamers will start a round with a shot goal in mind. They will play for a particular score per hole (birdie, eagle, etc.) and simply quit the round if they fail on a particular hole. They then start a new round, repeating the process until they finally achieve the low score they're looking for over the entire round. Tourney conditions may also factor in if they are from forward tees and/or with lower wind than the player normally plays.
2. Low tier players (hacks & amateurs) showing such low scores are usually players who have Master and above status but opened up a second (or third or fourth) account in violation of the TOS simply to game the system out of credits and boost their egos.
3. Yes, a few have cheats and hacks but I think the majority of low scores actually come from the players in 1 & 2.