There are multiple leagues for each rank and i assume they're mixed up after each season.
This way you're not stuck with the same associations over and over again.

Pretty much just like the ladder system in Starcraft works for example.

Losing a specific % of gained points is necessary, else everyone would sooner or later rank up to platin.
This is to make sure only teams of the same strength will face each other in the long run.

Remember that this is a brand new feature, it will need some seasons to sort itself.