Because of uncertainty, volatility, risks bitcoin keep flowing in and out but since it is valuating more than depreciating the money flowing in is way more than out, at the same time that if nobody was selling bitcoin it's value would sky rocket by lack of liquidity and huge spreads. So yes, people sell bitcoins because there is some source of influence making people do it, it may be emotionally or backed by some reasoning and this is why markets are always bull or bear, pump or dump !