I was adjusting front mech yesterday and just couldn't stop it from rubbing which was when I noticed the large chainring would veer off centre by the width of the chain or thereabouts.
At first I thought the crank must be bent our something like that which was when I spotted the loose chaining bolt. Or rather bolts, 4 of them. I was lucky that no bolts had disappeared entirely.
Clearly I had not tightened sufficiently when replacing small chainring a month ago.
All tightened up now with the help of a large flat screwdriver to hold rear in place. I found it easier to get the final tightness with chainset fitted to bike.
Anyway, let this be a cautionary tale of the dangers of not properly tightening chainring bolts