overflowing
衍 centers on water spreading beyond its bounds: overflowing, spilling over, or extending widely. The idea of excess or proliferation grows naturally from that image.
衍 combines 行 (going, path) with 氵 (water), suggesting water spreading along a path or overflowing its course. The exact historical development is uncertain, but the component combination fits the meaning of overflowing.
Picture water (氵) moving along a path (行) but spilling over the edges: 衍 is water overflowing its channel.
For エン, imagine an endless overflow of water: endless -> エン, and the water keeps spreading beyond its bounds.