If it went from 20% men to 80% men in a short period of time...no.
I'm not suggesting that the change was a statistical fluke. I'm suggesting that maybe in this case some women at Yahoo! were better management candidates than some of the men. In a ratio that's large but not unprecedented. Look at the rations found in other companies - so often its 80% men but that's not seen as unusual or wrong.
Sounds similar to the wage gap argument here: if it exists, where are all the companies hiring cheaper workers capable of doing the same work, exploiting the supposed market inefficiency?