Something that's startling is so unexpected that it shocks or surprises you. It would be startling to open your front door and see a clown standing there.
Startling events or circumstances aren't necessarily frightening, though they can be. A surprise party, if it's planned right, is startling, and it can be startling the first time you meet your best friend's identical twin sister. Alarm clocks, smoke alarms, and barking dogs can all be equally startling. They startle you — and startle comes from start, with its Old English root styrtan, "to leap up."