Why has my game decided to re-install itself?

1. What is the issue?

The update released today has made my entire 29gb HW2 game decide to completely reinstall itself. All 29 gigabytes. I noticed my connection while playing another game went a bit funny so went to look in My Games and Apps, only to see that for some reason I had a 29gb download going. I have slow internet so this pretty much guarantees I’m not going to be able to play for the better part of a week. This is the second time this has happened upon an update releasing for me, and it’s happened to my friends a few times as well. What’s causing the game to completely have to redownload itself? This is ridiculous considering that I’m not going to be able to play the new stuff for multiple days now.

2. What platform are you playing on?

Xbox One Digital Download.

3. What’s your gamertag?

BaconShelf.

4. What region are you playing from?

UK.