Brown sky pencil holly
I planted my sky pencil holly 3 years ago and this year part of it started turning brown. Why? I planted 4 total and the other 3 are fine.
Many hollies will get Black Root Rot if water stays around their roots too long due to planting too deep, poor drainage, too much rainfall/irrigation. If the plant isn't all brown you could take a root sample and a branch that has both green and browning leaves on the same branch to your local extension office for free testing.