Smurf #1: If the person is a bad - mediocre player then it'll show; they'll blame their team's loss on everyone else but themselves and will always run in 1v1 or 1v2 without assessing the circumstances first, and complain if they get ganked or lose the fight, saying such things as "LOL YOUR CHAMPION IS SO EASY TO PLAY", "LOL YOUR CHAMPION IS SO BALANCED" or "LOL YOU NEED 2-5 PEOPLE TO KILL ME".
These smurfers will generally have a bad attitude and cannot handle the fact they have more losses than wins on their main; because of their bad attitude there's a large chance they may have been banned before too.
Smurf #1.5: People who have been banned before.
Smurf #2: People who want to duo queue with a friend new to the game and don't want to queue a lvl 30 account with a lvl 1 - 10 who doesn't know how to play and has only been schooled on bot games so far. People sometimes solo queue with these accounts because they feel compelled to goof off at low levels and do the same thing as Smurf #3.
Smurf #3: People who are scared of using free week / new champs and want to try them out without being yelled at for playing them badly.
Smurf #4: People who just want to use troll builds for a laugh, but not necessarily throw the game. For instance I created an account so I could play as AD Ziggs. Won too much though so I got smurf detected and people started yelling at me for going AD Ziggs.
Smurf #5: People who want gimmicky names but don't want to spend the money and / or change their main account's name for whatever reason. I have a plethora of accounts named after the Metal Gear franchise on the US server. I also own Rumble Bot. However this is the only account I own on this server.
Smurf #6: These aren't actually smurfs, but because some people switch in between servers because different friends play on different servers; they'll need to create another account on another server and will obviously not be new to the game.
Smurf #7: The rare individual who's genuinely bored of high elo games where the meta is usually a passive 20 - 30 minute laning phase last hit whackamole game. Not saying all high elo games are like that, but a lot of them are. However this person can have habits similar to Smurf #1, and tend to come in packs and play in premades with others like themselves. Not to mention the queue times are ridiculously long.