Re: Any lawyers here? If so, I have a question.
I'll chime in as both a CPA and an attorney. Won't comment on their changing the W-4 on their own, though... BUT, if her exemptions went from 0-10, then, yes, less would be withheld. No clue why they'd do that, though. But, hey, if you don't owe any penalty with it, then you got to have use of that money all year instead of the government having it.
Another reason you might owe more... check your income as compared to prior year. Check your deductions as compared to prior. Did you leave anything out? Are you being limited whereas you weren't before? Have a kid turn 17? If so, bye bye child tax $1000 credit for that kid.
LOTS of reasons why you'd owe this year over last, most, if not all of which, have nothing to do with that change to her W-4.