There is a new bug where if you place a two sticky pistons on top of each other, facing the same direction horizontally and place a redstone block on the above piston sticky side, the bottom one extends.
MC-173602
Redstone piston bug
Resolved
Duplicate
0
Msmg2
(Unassigned)
Unconfirmed
Redstone
piston
20w09a
Retrieved
Thats normal, the redstoneblock powers the piston at the bottom but the piston needs a blockupdate to "realize" it. The Blockupdate is caused by the piston above.