Got placed in tourneys higher than my rank

So I've been playing RL since 2015, but I've been consistantly crap.. I play weekly, but I don't grind, nor feel the need to. I've been riding steady at d2/d3 at 2s for years, but since the "recent" nerfing of the mmr system, I'm at p2/p3. This season wasn't different; haven't reached diamond. Now all of a sudden I'm participating in diamond tourneys.. Anyone know why I get placed there, when all my ranks are plat at best?