Presumably, no primer was applied to the wall and the plaster is, as they say, burnt. That means the plaster lost its water too quickly (due to the absorbent Poroton), which caused it to form only a weak or no bond with the wall.
The deep primer or bonding agent prevents this. Always treat Poroton bricks with it beforehand.
You can test this yourself. Just pour a glass of water onto the Poroton. It soaks in immediately. And that’s exactly what happens to the plaster. It dries before it can bond to the wall.
No mesh was embedded either, which I would have done especially with gypsum plaster. But that’s a matter of opinion.
However, as you can see, the installer already knocked on it quite a bit with a hammer. Still, the plaster should not be that easy to remove without residue.