The local surface tension of the foam decreases, causing the foam to burst. 

Someone has studied the defoaming process of polysiloxane in the oil system. They took continuous pictures of the foam system at a speed of 1/1000 seconds. When the drop reaches the bubble film, the surface of the bubble film breaks and merges into large bubbles. From 1 to 3, the bubbles are merged from 4 into 1 big bubble, and the bubble film becomes thinner. Finally, the gas and liquid separated rapidly, the large bubbles disappeared, and the bubble bursting ended when the bubble was 4 times. This also explains objectively why a series of bubbles burst when a drop of defoamer goes down.

When the defoamer adheres to the bubble film, it is immersed in the bubble film liquid, which will significantly reduce the surface tension there. Because in the water system, the solubility of the active ingredients of the defoamer to water is small, the reduction of surface tension is limited to the part of the bubble film, and the surface tension around the bubble film hardly changes. The part with reduced surface tension is intensively pulled and stretched around, which finally causes the bubble to burst.