Underpayment penalty on taxes......Paging Poodle...please see last post

I got back our income tax report this morning from the accountant and to say I was floored is an understatement. For openers, we had twin girls this year, so I figured our liability would be waaaaay down. Not the case, we actually owe more this year than last.

First question, while on maternity leave, my wife received short term disability, which the accountant report stated that it was not subject to withholding (which I'm assuming means in lay terms, no taxes were taken from it, so that amount is owed in lump fashion come April 15)?

Second question, he also noted that we incurred a underpayment penalty for not having enough withheld or paid via estimates. Is this due to the short term disability? Is this a mechanism in place to keep people from claiming 0, taking their entire paycheck up front, and then owing the entire tax bill on April 15?