Many newer digital cameras (both dSLR and Point & Shoot digicams) have a built-in orientation sensor. Virtually all Canon and Nikon digital cameras have an orientation sensor. The output of this sensor is used to set the EXIF orientation flag in the image file’s metatdata to reflect the positioning of the camera with respect to the ground.
By the way Dropbox will never auto-rotate a picture automatically.
Either you did it in your Dropbox local before uploading in your server or you have the exif property auto-rotate 90 enabled in some pictures (see pict meta-data )
I think it s possible to avoid exif property in all files.
JHEAD -v -autorot C:pictures***.jpg
The exif property may be read in a browser differently than on a software Pc program.
Hope it will help a little bit.