A few reasons for why they see a point to do it.
1) Deliberately loosing with the view of loosing so many games with a lot of EXP that they can rank others up faster, usually when ranking others up it may result in payment of monetary value.
2) They simply want to piss other people off and derank with the view of getting a angered response from a fellow team-mate.
3) They simply do not wish to be at their level anymore and derank with a view of loose their current rank.