Originally Posted by Flexor
Yes, it sounds like it is caused by a natural protraction of the shoulder blades which occurs when you press when leaning backwards or lying down. Notice that if you do weighted pressups, your serratus will ache like hell because the mechanics are such that there is a lot force needed to protract your shoulder blades. Bench presses do not have this same effect because they are open chain. The closed chain pressups mean you have a more rigid platform and the serratus is involved more.
This means your hands are staying still and you are applying force to the ground rather than a weight, whereas in bench presses you stay still and your hands move to overcome the weight. Closed chain alters muscle recruitment patterns and increases stability.