The most common way for leagues to be created is by friends lists - my league is mainly 5* and 6* teams and the same 6 or 7 teams have been together for 5 seasons now, we like it that way. I did this by adding ppl I met in the Cup and later stages of CL. (I did this after I had a 'no friends' random league and was bored to bits with the undefeated season.)

Sometimes you having just 1 or 2 particular friends, who then have many mutual friends, is enough to pull you into the league.

Same thing works for low level teams, if you add friends of same or lower quality then you'll be paired with them if there are enough common/mutual friends to strengthen the link.

If you have no same level friends you'll get a random league.

So you're either are being pulled together with friends or friends of friends or both.

PS. The closer the links between friends the more likely you are to be put together. We have an OMA league where 16 teams were created on the same server over 2 leagues - 2 lots of 7 promoted teams were them pulled together into 1 league in Season 2 because all the teams only had those friends and no others.