Why did SPY fell today on the open and then reversed massively on no catalyst?

Hello,

I am trying to get the sense of todays price movement besides markets are irrational etc. I hope to find logic behind large moves so it helps me to execute better trades. I expected that we rally on bad GDP data since WH has downplayed the report and prepared the market for bad numbers. Was the dropdown caused by profit taking from yesterdays rally after FOMC statement or am I missing something else?