Minecraft wiki entry for Redstone_mechanics, associated text: In Java Edition, pistons, dispensers, droppers, and can also be activated if one of the methods above would activate a mechanism component in the block above the component, even if there is no mechanism component there (even if the block above the component is air or a transparent block). This rule is often simplified to saying that the components can be powered by blocks diagonally above or two blocks above, but other methods of such activation exist (see image to the right). This method of activation is known as quasi-connectivity because the mechanism component's activation is somewhat connected to the space above it.
Doors occupy two spaces, one above the other, and anything that activates either space also activates the other.