The layers of onions are the bases of the leaves that once stood green above the soil. The outer layers are old leaves that continuously get replaced by inner, younger leaves. As they die, the oldest leaves return most of their material to the base of the bulb, recycling their resources for the living parts of the plant. Only the “onionskin” remains as protection. The inner layers of the onion bulb are young leaves and buds. They swell as they grow, stretching the outer leaves, and the further out the leaves, the more they must stretch.

Now, onion tissue is made up of cellulose chambers filled with living fluid forced in under pressure, stretching the cell walls, so that the tissue is like a lot of taut balloons stuck together, not flabby at all. Pop some of the balloons, and tension in the rest of the structure pulls it lopsided. Similarly with the onion leaves. The older, outer leaves are stretched evenly, but when you cut off the tops and root bases the outer leaves no longer get stretched lengthwise, only sideways; they shrink elastically, leaving the inner leaves sticking out. A cut at right angles would cause a different pattern of distortion.