Because of the Holiday season, not only in Our Country But all over the World.
One thing I have noticed that in recent times (After 2012) on EDX if a movie is carrying just about good WoM it collects Huge.It means that People come out in Big Numbers if there is something for them. A movie like 3 Idiots or Bajrangi Bhaijaan can Cross 400 cr mIark easily even if the 2 Big Holidays fall on Sunday (in case of Don 2/ Dangal).
I have watched Dangal.It's a very good movie But certainly Not The Best Of The Decade. I'm saying this with 100% Honesty and Majority will agree with me.Also the Year 2016 has been very Poor for the Bollywood and Dangal (without a doubt the best film of the year) came at the Right Time.Also the Ticket Prices are Hiked during Xmas,For instance, Sultan ATP is approx Rs 92 (after Tax Deduction) While Dangal ATP will be Rs 100+ (after Tax Deduction).
In PK there was nothing Except Aamir Khan classic acting.Infact OMG was better than PK. But it was entertaining and was having Universal appeal Thus it scored Huge at BO or I should say 'Over-performed'.