Demoted with 100 LP without passing through 0 LP ?

I lost a match and I was demoted from diamond 1 at 0 LP to D2. Then I lost the next match (position 5th) and I was demoted from D2 (75 LP) to D3 (75 LP).

How is possible to be demoted with 100 LP in one step, without passing through 0 LP ?

And why I lost 100 LP if I finished 5th position ?