They explain it very clearly.

My take was that his wife had taken a part time job without his knowledge, then left him with the tax issues.
There was talk during the film about her being disloyal and a bad woman, and John was portrayed as a real straight arrow- even Goldman said that he wouldn't have cheated on his taxes. ("He's as straight as a grizzly's dick.")
"Two little mice fell in a bucket of cream"

If I remember correctly, John Gustafson was claiming Union Health benefits as a married man for 20 years, but he didn't mention that for 11 of those 20 years, his wife had a part-time job, which meant that John's benefits would have been subject to more federal tax $13,000, which accrued penalties over the years from lack of payment, bringing it to $57,000.
As to whether it was deliberate, who can say? The federal tax system is a tricky one to fully understand, and the omission of that fact may have been accidental.