see, if you carefully observe the mechanism then you'll see that the very 1st step is the abstraction of alpha hydrogen by a strong base.

 

now, for the abstraction of the alpha hydrogen, the alpha hydrogen must be acidic else it will not be abstracted.

 

in the question given by you, the alpha carbon has 2 methly groups attached to it, so, the methyl groups are giving their +I effect thereby decreasing the acidity of the alpha hydrogen.

 

now, since the alpha hydrogen is no more acidic then the OH^- ion will act as a nucleophile rather than a base & so, there will be attack of OH^- as a nucleophile & the reaction will proceed for canizzaro.

 

I hope you get the concept...