Small Grid Pistons and/or Conveyor Collision Boxes
It seems either small grid conveyor and/or piston collision boxes are incorrect and are too large, causing pistons along side conveyors to interact and causing a build up of force in the structure that can induce explosive behaviour. This issue is not present with large grid variants and thus is considered unexpected behaviour.
Attached are two images, one small grid, one large. The issue is not greatly apparent in screenshots but easy enough to recreate. I have also included a link to a save to expedite investigation.