Exposure bracketing changes ISO and shutter speed in aperture priority and other modes. However, shutter speed, aperture and ISO are the same when shooting with exposure bracketing in shutter priority mode. How did that happen? I'm using a Canon EOS 100D camera.
I would expect the camera to alter exposure using the aperture when on shutter priority mode, whether for bracketing, exposure compensation, or just because the lighting changed. If in auto iso mode it might also use the iso, but I would expect it to be primarily using the aperture.
Did the shots you took come out looking as you would expect for bracketed shots, ie sequences of brighter and darker exposures?
There are up to 4 exposure parameters which can vary. When you fix one or more, the camera can vary any of the other ones. If you end up fixing all 4, then the camera will override one of them and different models do not all behave the same. In some cameras, there is even an option which lets you specify which parameter is varied.
Supposed you fixed shutter-speed and ISO, then the camera can vary the aperture to give different exposures. However it can also vary flash output if you are using the built-in flash or external TTL unit. Some cameras give you the choice by having an option specify Ambient Only or Ambient and Foreground. For the former, aperture will change, for the flash output will change unless it is at its limit, in which case the aperture will change anyway.