1) When equal wt.s are put the arms are slowly brought to equilibrium and then the arms become horizontal. If the arms are not horizontal, even though the wt.s are equal the torque's do not become equal. So there is a swing.

 

2) When equal wt.s are put in both the pans, the arms should be horizontal. But if you drop one wt. in one pan and put the other wt. into the other pan, due to impact the arms may swing up and down.