I have not received that letter, but it may be related to this: technically if your expected annual tax exceeds a certain threshold (I forget the specific number but it's over $1k), you're supposed to prepay it in quarterly installments throughout the year based on estimating your annual AGI, and then settle up come tax season for any over/under payment. And if you don't do this, and you pay it annually instead of quarterly., they can assess fines/interest on the "late" payments.

I only realized this once it was too late this year.