The way I see it... If you have a weak team which some 35T players will do OR having a poor season in the league without any real hope of getting in next seasons CL. Then the best option is to tank since your not really giving much up in doing so since you've pretty much 'lost' already, You may as well forfeit promotion and retain your players stars for a better chance the for the following seasons.

Also situations in your server may also cause you to think about tanking for a while. Such as the 3* or 4* transfer market or being at the top of your server and having to play against a lot of heavy token buyers each season in which your probably going to lose there as well.

The main reason many players tank in this game is not to lose on purpose but to improve their playing experience in this game in the future. I've been tanking on Capelli Tigers for 2 seasons because of these reasons of being near the top of the server as well as not being drawn in the same league alongside a friend.