Probably cause people are selling once the price goes up a little thinking that is when they are gonna make profit. Also, they believe that the price won't go as high or close to it like last year anymore so they sell all their assets and cause a massive fluctuation.
but many are waiting until the end of this year. many of the bitcoiner's hopes that the end of this year will increase significantly. but not a few also panic sell, which worsens the current situation