I had much less tax withheld this year compared to last, even though my income has increased a little bit. What could account for this change in withholding?

You would need to discuss this with your employer.  

 

The amount withheld from your paycheck is typically based upon your W-4- Employee withholding certificate.

 

W-4 Form

 

If you ended up owing taxes with your return, you may want to consider updating your W-4 to ensure enough tax is taken out with each paycheck so you avoid having a liability when you file your return next year.

 

you should be asking this question 14 months ago, not now.