Are you blending something hot? When you blend a hot item the steam that is released will increase the pressure in the headspace above the contents in the blender. The lid will eventually blow off to release the pressure.

To avoid this problem begin blending with no more that one cup of ingredients. Before turning the blender on, remove the center lid insert to allow venting of steam. Blend on low speed. As you blend you can add another 1 1/2 cups of additional liquid through the opening in the lid.

Trust this method will resolve your problem.