This is actually pretty simple. Yeast needs O2 in order to reproduce. You need that to happen so you have enough yeast to digest all the fermentable sugars in the wort. Once the O2 is depleted, the yeast pretty much stop reproduction and begin eating the sugars. By the time you rack to secondary, the yeast are pretty much dormant. Any O2 introduced at that time will not be utilized by the yeast because there is little or no food and as such no reproduction will take place. The O2 then binds with compounds in the beer causing a taste similar to that of wet cardboard. Not a flavor you want in a beer. A little oxidation does work in some strong ales such as barleywine, old ales, and some strong belgian ales. That, however comes from long aging and gradual seepage of air into the bottles and from the small amounts of air inadvertently introduced during bottling and other handling.
Hope this helps.
Bugeater Brewing Company