Major product in reaction of butan-2-one + selenium dioxide
To understand the major factor we have to consider the mechanism of the reaction first. The reaction you mentioned is popularly know as Riley oxidation. The mechanism is as follows:

As you can observe the RDS of the reaction is the tautomerization of the keto form into an enol. Thus the extent of enolization determines the major product in this reaction.
As the reaction takes place in an neutral environment the acidity of the alpha-hydrogen plays the major role in deciding the major enol formed and so the corresponding oxidized product is major.
In short, the major product in Riley oxidation is the product associated with the major enol content which in the given conditions is determined by the acidity of respective alpha-hydrogens.