I thought that too. There's a bit of dried mud but nothing that looks like it's preventing the bolts from pinching. That doesn't explain why it fell off the first time when the end cap was in place and there's been no movement for the last couple of years.
I assumed something had worn out, I guess not.
If consensus is that the bolts are not holding, I can get them tight and they seemto work themselves loose, then a spot of threadlock will do the job.
EDIT: I'll have a look for and end cap and see what happens, thanks.