My BJCP Course states that (too) late aeration can cause diacetyl. My understanding is that diacetyl is produced by yeast during the first phase of fermentation. How could too much oxygen in a finished beer cause diacetyl flavor. Is it due to oxygenation?
Even if all the yeast is filtered out of the beer, if there is diacetyl precursor still in solution (alpha acetolactate) the general oxidation of the beer will cause the alpha acetolactate to be converted to diacetyl. This is a non-enzymic, yeast-less process. Minimizing oxygen minimizes this affect, as does minimizing alpha acetolactate content.
Look here, under "Beer Aging" section of article.
Actually I found an article from BYO that says:
Uptake of oxygen late in fermentation leads to excessive diacetyl production. Yeast will begin releasing more alpha acetolactate if oxygen is introduced late in the fermentation. This eventually leads to an increase in diacetyl, so care must be taken to avoid oxygen pickup during beer transfers.
My understanding from the reading I'm doing is the extra oxygen will cause the yeast to produce more diacetyl. If the oxygen is added early on, the yeast has plenty of time to clean up this extra diacetyl. If the oxygen is added late in fermentation, the yeast has time to make the diacetyl, but not clean it up.
Here's the Google search I used. Some of the links may be useful.