After activation, the wooden crate that the stargate was stored in has circular holes on both sides. The vortex that would create these exits only on one side of the ring. Or at least it usually looks like it. In the first motion picture, however, the vortex wobbles back and forth before stabilizing. In this movie the gate is shown head-on, so it's impossible to say what's going on behind it. But even if the vortex exists only in front, it may cause air disturbances strong enough to destroy the crate wall right behind it. There is no specific claim in the series that standing right behind a gate being activated is perfectly safe.