I'm not sure what you would gain by having a CPA go through your pay checks at $300 or $400/hr (or whatever they charge).

For some unknown reason, your employer under-withheld taxes. Your best recourse would be to work out a payment plan with the IRS and have your employer withhold additional money each paycheck. It sucks but a lot of us have been there.

I'm sure it has something to do with your bonus and pay raise if you didn't have the same problem last year. There are a lot of posts on this sub about the exact opposite problem, where their employer withheld taxes at too high of a rate after overt time or for a bonus. It's why so many people believe they're losing money by working OT or getting a Christmas bonus.