Underpayment penalties are assessed if you don't withhold or pay enough tax on income received during each quarter. It usually happens when you have a balance due. You can investigate by checking if your 2018 balance due is less than $1,000
or if you paid at least 90% of the tax for the current year or 100% of the tax shown on the return for the prior year, whichever is smaller."
See more information here:
https://ttlc.intuit.com/replies/7250569
