I reckon a physical attack makes the shield simply shatter without expending the expect amount of energy, per se. So, it just disturbs the structure of the shield. We know the shields are kind of flawed as they are, in a certain sense, since merely creating/maintaining them without deflecting attacks already eats energy. Perhaps the shields are structurally utterly incapable of dealing with a physical object interacting with them. At least a physical object any more solid than gas, unless it's the atmosphere that already makes them inefficient normally.