@TBOY  
The types of files created when you use a scanner or digital camera sometimes have personal information attached to them, like location, owner name, model registration number, computer owner, etc… To make sure that none of that reaches the Internet, many websites wipe the file of any information that the scanner/camera added. Unfortunately, this sometimes includes user-manipulated bits of the file, such as how the uploader had rotated the file before adding it, and now it is sideways here.

@Barhandar  
What?

Derpibooru erases EXIF data (for your own privacy), this includes display rotation.