By default the drop-down menu displays above the button when there is not enough visible space below the button. You can force the drop-"down" menu to always appear downward. You can also force the menu to appear upward.

var a2a_config = a2a_config || {};
a2a_config.orientation = "down";

var a2a_config = a2a_config || {};
a2a_config.orientation = "up";