Because of the weight of the ends of the forks, and how they’re distributed behind the penny (closer to the glass), the center of gravity of the whole system is actually shifted quite significantly. If I’m right, it would actually have to be right where the penny meets the glass. This mean, in a sense, all the “weight” of the system of the forks and penny is resting right on that point, rather than out in the air, so if you balance it, it’ll be stable on the glass.
Bingo. The mass is placed perfectly displaced so that the center of gravity allows it to balance on the penny.