In the 2011 world cup teams fielded 22 players each and in the 2015 world cup fielded 23 players. When was this changed? Is there a reason given by world rugby as to why this decision was made?
This was a change designed to reduce the number of matches which were forced to have uncontested scrums due to not having appropriate players available - rugby union scrummaging is a highly technical activity, and having players not skilled in it attempt it is not allowed due to the risk of injuries involved if the scrum collapses.
The original change to the Laws happened in 2009. However, at that point the option to allow 23 players, which had to include 6 front-row players, was an option only for national and cross-border matches. It explicitly did not apply to international matches at that stage, which is why the 2011 World Cup was played with 22 man squads.
I can't find the exact date when 23 player squads were allowed in international rugby, but the change had obviously happened by 2015.