Why water side cock is opened first & then steam side cock to blow down a gauge glass?

The main cause is the enthalpy (heat energy) of steam is greater than enthalpy of water according to thermodynamic chart. Gauge glass is an enclosed area and it cannot withstand high pressure more than 34 kg/cm2. When high enthalpy is applied in an enclosed gauge glass, there is a chance of rupture or fracture. So it is wise to open water side cock first during gauge glass blow down.