Or HaChaim on Numbers 10:6/אור החיים על במדבר י׳:ו׳

1 ותקעתם תרועה שנית, "and you will blow a second Teruah, (alarm) etc." One blast of the trumpets did not suffice to cause all the tribes to start moving. Sifri 2,73 states that the members of the clan of Gershon were occupied in dismantling the Tabernacle and had started moving before the tribe of Reuven. This made it necessary to blow the trumpets again to give a signal to the tribe of Reuven, situated in the South, to start moving following the clan of Gershon behind the components of the Tabernacle.