Why is it that when UA has a change in flights or times of flights - when they make the changes it throws the whole Itinerary off kilter!

I am booked RSW/ORD/BKK 11/30 and return BKK./ORD./RSW January 9th

The Flt from ORD to RSW (Flt 1206) January 9th has been cancelled. (This is odd to start with)... When the changes were made
I was taken off Flt 881 from ORD to Narita (Upper Deck 16A) and put on 883 ( A 777) - This Flight was not involved in the changes. Then on the return everything fine 'til I get to ORD (Flt 882 arr. 2:20PM)... the new ORD-RSW Flight was shown as routing through Washington (IAD) (Flt 936) but the flight shows as leaving at 1:15PM...!! I am already over an hour late for this Flight!!!

What causes all this when changes are made to an Itinerary? Has anyone else suffered this problem? Thanks Hayjon