Noticed with marble cobble, haven't tested with other physics blocks, occurs when it passes over a ladder, not just when landing on a solid block below it (breaks even if air below the ladder)

From testing: happens with all gravity blocks (cobble, dirt, etc.)

Other weird behavior:

• pops off on a trapdoor on the top or side of a block (even when placed, not falling, on a trapdoor placed on the top of a block)
• destroys a trapdoor if it's on the bottom face of a block
• pops off on a door
• pops off on a piston extension to the side
• lands on a piston extended upwards
• doesn't destroy webs, but takes long enough to fall through them that it pops off due to the timer expiring
• pops of on a fence gate or fence when falling from > 1 block high, but breaks it when placed immediately above
• pops off on glass panes from any distance