Have you ever tried taking a photo of a building only to have the building look like it’s leaning backwards? That’s a perspective problem known as keystoning, and it’s caused by the building and the camera lens being on two different angles.
Usually, unless you’re fond of climbing trees or you have a friend at the fire department, standing on the ground and tilting the camera back a bit is the only way to get the top of a tall building into the photo, and since the camera is tilted away from the building when you snap the image, with the top of the lens being further away from it than the bottom of the lens, the building ends up looking like its leaning backwards. This problem actually exists any time you need to tilt the camera back to get something tall into the photo but it’s most noticeable with buildings due to their straight parallel sides.