Piston gets activated when placing any block next to it

When placing 2 pistons each above, and activating the upper one (see screen), only the upper piston gets activated. ( as expected ).

But if I delete, or place a block next to the lower piston, it also gets activated. But only when placing/removing.. After reactivation of redstone, only the upper piston gets activated, unless you place/remove any block next to the lower piston again.