This is what we call taking profit (I am taking massive withdrawal as massive selling of BTC due to the word dip). People who invested in Bitcoin when it was around cheaper are securing their profit. Aside from that the automatic dip experienced after the Bitcoin price surge is often times called a correction. Since Bitcoin is overpriced due to hype and positive emotions in the market, the price needs to be stabilized somewhere to be able to establish the average trading rate. It is obvious that when a large supply is being sold, the market will experience retracement since demand is outweighed by the incoming supply.
Since Bitcoin is an open market, the price often swings wildly and this is the reason why the Bitcoin market is considered highly volatile.
Not entirely true, people or investors have their target price, so it is normal for them to sell when they meet their selling price. Only greedy people will regret after selling their stash at their set target price since they don't have contentment and always want to get the highest possible profit.