Since March 15th in 2015 all shops in Hungary had to remain closed on Sundays, with a few exceptions. This law however did not last too long, and as of April 17th 2016 shops are free to stay open on Sundays again.
The entire matter of closed and open stores on Sundays has had 100% to do with politics, and it was politics that led the current leaders of the nation to go back on their decision and again let shops stay open on Sundays. There are people on both sides of the political world and normal Hungarians who are really happy about it, while both workers and other people are sad that stores will re-open on Sundays again.
Most shops might not open already from Sunday April 17th, because it takes quite a lot of work to reschedule everything, make new working plans and so much more, but as a tourist coming to Budapest you can expect to see more and more shops open on Sundays again.