Frost damage usually, blows the front of the brick off because it's porous and freezes. You can cut that brick out and patch a good brick in, it would be unusual for it to be covered in that kind of problem. I suppose it's not double brick though. Not picking at the intention, I'm sure you've weighed it up.
It looks like it's been patched pretty heavily in the past, there's definitely some movement that's gone on too. I'm planning to do a green roof so wouldn't trust it to hold the extra weight / not fall over and crush someone.
Frost damage usually, blows the front of the brick off because it's porous and freezes. You can cut that brick out and patch a good brick in, it would be unusual for it to be covered in that kind of problem. I suppose it's not double brick though. Not picking at the intention, I'm sure you've weighed it up.