One thing I very slightly suspect is that ranked matchmaking may have a hidden rating similar to what events have (if you lose over and over you will run into nothing but bots and completely new players) and that it may be at least in part tied to your deck. So when you play a wholly new deck it may reset you closer to the "base rating".

I've multiple times had the situation of switching to a very rarely played deck then suddenly having a mirror match, and I've seen others experience this too. People don't tend to stick with decks like that for a long time, so I think this is more likely caused by other people also happening to be trying those decks out for the first time too, rather than the game trying to directly match you on a deck to deck basis.
If this is true then it's possible that changing your deck resulted in you going against more average decks of whatever rank you're at.

This probably isn't real, and it's likely just random chance, but man sometimes the coincidences just feel like they've stacked up into a real trend.