Why? Because the risk now is about twice as big as when you went long as the inside candle was broken to the upside.
As volatility expands the only thing we can do to avoid the risk of ruin is to adjust position size to vol and account size.
In order to finance some of the risk it is advisable to take partials off as you come into profit. The goal is to get a free look at the market and to play with the House's money. You can do that by squaring half after your profit is as big as the risk you initially took on. You can, of course, be more aggressive in protecting your downside but that is a discretionary call.