when and why was Rwanda seperated from the rest of Belgian's congo
colony and declared an independent state, while the rest of the
territory became one state?

This link might interest you

http://news.bbc.co.uk/2/hi/africa/1070329.stm

also this one

http://americanradioworks.publicradio.org/features/justiceontrial/rwanda_chronology.html

I think that the short answer is that Rwanda was never part of the
Belgium Congo, it was part of German East Africa that got taken over
by Belgium during WWI

As already pointed out by frde-ga Rwanda was part of German East
Africa which was occupied by Belgium and Britain in 1916.

Some of the events following the occupation in 1916:

May 1916 - Sep 1916        
Belgian conquest of parts of German East Africa; Ruanda (May 1916),
Urundi (Jun 1916), Usui (Jun 1916-Mar 1921), the region of Ujiji (Jul
1916-Mar 1921), and the region of Tabora (Sep 1916-Feb 1917)and also
other parts.

30 May 1919
Treaty with Britain: only Ruanda and Urundi remain under Belgian administration.

20 Jul 1922
League of Nations Mandate of Ruanda-Urundi.

1 Mar 1926 - 30 Jun 1960
Administrative union with Belgian Congo
http://www.worldstatesmen.org/Burundi.html#Ruanda-Urundi

when on 1 Jul 1960 the Congo gained independence
http://www.worldstatesmen.org/Congo-Kinshasa.html