gravity affected blocks breaking when it hits other gravity affected blocks

when you have 2 sticky pistons on top of each other with 2 blocks tall gravel on top and you retract the top piston first and then the bottom piston the top piston pulls the first layer of gravel down and gravity makes the 2nd layer fall but when the 2nd layer hits the first layer of gravel it breaks this only happens when a sticky piston pulls the first layer down and it works with all blocks affected by gravity