Minecraft wiki entry for Quasi-connectivity, associated text: Quasi-connectivity is a property of dispensers, droppers, and pistons that allows them to be activated by anything that would activate the space above them, no matter what is actually in that space. While quasi-connectivity can be difficult to work around sometimes and might seem like a bug, it is officially recognized as a feature that "works as intended" and does make some builds much easier (for example, piston walls).