It is because the picture on the left above it is taller than the one on the right, so the 'big noise mpc' is floating left against the sliver of the 'rosie's ny pizza' article that is sticking out below the one on the right.
You need to make the bottom of those two images flush, so that the image below it will float correctly to the left. This should fix it. This image can describe it better than I can with words. https://i.stack.imgur.com/kie2q.jpg
This code should fix it:
.portfolio article:nth-child(4) { margin-bottom: 17px; }
'