The reason for this phenomenon is that the rate of change in the upward acceleration in prices slows considerably. Prices are still going up, but not as fast. Once the rate of increase in prices slows, the longer term moving average (26-day) starts to catch up to the shorter term EMA (9-day). Therefore, the difference between these two moving averages decreases. Hence, the decline in the MACD lines.

If you eyeball the 13-day and 50-day EMA lines on the price graph, you can see that on April 1 (or thereabout), the purple line (which represents the 50-day EMA) starts to close in on the light green line (which represents the 13-day EMA). MACD uses moving averages of different lengths than these, but the effect is the same.

So, when a bullish MACD green bar stops it doesn't mean that you should bail. It does mean that it's time to start paying closer attention. You don't want to get shaken out during normal retracement action. Normal and healthy retracement action will stop a bullish green bar, but that doesn't mean you have to exit a position, only to see prices resume rising after you get out.

I usually wait for the MACD lines to dip below the centerline before getting completely out. The end of a MACD green bar might be a good time to lighten the load and take some profits, but it doesn't necessarily call for dumping out of your entire position.