There might be other explanations, for example WoW’s matchmaking starts off at certain parameters that’s ± of your own MMR, and then expands gradually the more time you spend in the queue.
It’s hard to say what causes it without looking at everyone’s match histories and personal MMRs.
It’s very unclear how streaks affects the rating system in shuffle. Because it counts each round as a match in shuffle, and then it just shows the summaries of all the rating won and lost in the shuffle to players once it’s over (which has caused a lot of confusion for the player base), and because you can see your own CR and MMR is unchanged throughout the shuffle, it means it’s probably throwing both the “streaks” of the rounds you lost, as well as the rounds you won, all at once when the shuffle ends.
So how those “streaks” affects the rating stability in shuffle’s rating system, is very unclear.
As mentioned in the paragraph above, it’s very unclear how it works in shuffle.
For the record though, I think this system is unviable for the shuffle game mode, and Blizzard needs to make some big changes.
Suggestion to fix the rating system in shuffle < That’s how I think they can fix the rating system used in shuffle.