bitcoin has a 10 year history and if you check its charts you can see that this type of "rollercoaster" ride where price shoots up to the moon (a bubble) then drops down (the bubble burst) has been happening from the very first days when bitcoin was listed on the very first exchange platform.

so to answer your question, the reason is the same as when it decreased after reaching $0.1 and it is the same as when it decreased after reaching $1, ... after $2... after $10.... after $100, ... after $200... after $1200, after $20,000 and in the future the same reason why it is going to decrease after reaching $100,000, and so on.