1- The thinset mortar was mixed with too much water and gravity caused it to slide. (Most likely cause)
2- It was walked on by a pet or person.
Mortar should be mixed to the consistency of peanut butter. In other words it should be sticky and hold its form.
Tile joints should be kept straight with a level or straight edge and I would recommend the use of spacers on each corner of your tile. These will help keep your joints the desired width and help to prevent sliding. A fan is also useful to speed up the drying process.