Oxidation of alkenes by selenium dioxide to diketones

I know that $$\ce{SeO2}$$ readily affects the allylic position. The reaction mechanism of this reaction goes like this:

(Source: Wikipedia; License: Public Domain)

But, in my book, there's another reaction given as an exception:

Here, an $$\alpha$$-diketone is formed.

Why and how is this alternative product formed? What is the mechanism for this product?

• $\ce{\alpha}$-diketones are often formed when $\ce{SeO2}$ reacts with alkynes that don't have any hydrogens on the carbons adjacent to the acetylenic carbons. Are you sure your book showed an alkene? – ron Nov 1 '15 at 23:32
• Yes, for sure, I have checked it again and its an alkene not alkyne. I know that alkyne reaction but this is not that. – shaistha Nov 2 '15 at 8:23
• I found formation of an $\alpha$-diketone from a ketone, but not from an alkene. – TAR86 Mar 13 at 5:54