If you have a single object that has a net charge on it (say excess electrons), then these charges move around in the object if there is a net force on the electrons (from other electrons). These charges eventually reach an equilibrium where the net forces on each electron is zero.
In particular the net force of the electrons on the surface of the object is also zero. But looking at one electron on the surface, you see that there may be interior forces that are pushing the electron away to the surface, and would succeed in popping the electron out if there were enough force on that electron to help it escape from the atomic forces that are holding it in the object.
If the net force on this surface electron were not perpendicular to the surface, then that electron would begin to move. Since you are told that the electron is not moving, then the sum of the electron forces on this single electron must be normal to the surface. If the net force on this electron becomes large enough (e.g., add another object with an opposite charge nearby), then you get a static discharge through the resistive air.
