From the Super Mario Wiki
The Pushing Blocks are items from Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3. They are pinkish-orange blocks that move when pushed. When they hit a wall, they squish against the wall, growing taller and thinner. When Ground Pounded on, the inverse happens. This is an easy way to get items over spikes. They are very rare, and only appear in two levels. In the Game Boy Advance remake, there is a clear Pushing Block in Endless World of Yoshis, bringing the total to three.