Sounds like oxidation. Causes and effects of oxidation are all over the place.... old malt, old extract, aeration at various points and how it was done, how the fermenters are handled, bulk aging prior to bottling/kegging, air intake during bottling or in the keg, just plain age of the beer itself..... or could also be simulated by over-use of crystal/caramel malts......... could be any number of things.
And I always hate to say it, but it truly could be a sanitation or contamination problem. It happens even to the best brewers sometimes. Don't think it's not possible, because it IS. Or the yeast might not have been as healthy as you thought, and something else got in there. Happens.