Tunguska
They were flying around our Solar System out of curiosity, far away from their home. They were sightseeing, and amazed with all that they saw forgot to check up on their fuel supply. When they noticed that they were running low, they decided to refuel on one of the local worlds. Like most advanced civilizations in our Galaxy they were using nuclear fusion as the source of energy, and all they needed was a large supply of hydrogen and they would be fine again. They headed for the lake Baikal, the largest reservoir of fresh water on Earth. Unfortunately, right before they were able to finish their descent, something went wrong with the spaceship and it violently swerved northward. The spaceship exploded over the region of Tunguska in Siberia, an explosion that was felt thousands of miles away. This came to be known as the Tunguska event and it took place on June 30th 1908, exactly 100 years ago. The crew of the ship managed to escape just moments before the explosion took place, and they and their descendants over the course of following decades made their way to the western Balkans, adopting to the local culture as much as possible, and retaining the name of the region in which they landed as their adopted surname. And that, in a nutshell, is how my ancestors came to Earth.