Rob Kardashian -- The Photog Was a Trespassing Peeping Tom!

9:59 AM PT: An eyewitness tells TMZ the photog had actually driven her car inside the garage and was lying in wait when Rob started changing and then suddenly leaned out of the car window to take the pictures.

Rob Kardashian stole a photog's camera memory card Wednesday because she dared to trespass on private property where he was changing his clothes ... at least that's what Rob is telling friends.

TMZ broke the story ... Rob manhandled the photog and snatched her camera's memory card from her. The cops have launched a robbery investigation.

As for why Rob did it ... he's telling people he had just gone to a private, West Hollywood gym owned by famed trainer Gunnar Peterson.

Rob is telling friends he was changing into his gym clothes inside the garage ... in an area that is not visible from the street. Rob claims the photog somehow entered the garage -- which is caged off to the public -- members must get out of their car and enter the code to get inside.

Rob says as he was changing, he noticed the photog inside the garage, snapping pics. It's unclear how the photog got through the cage.

Someone who manages the gym told TMZ ... they train big celebs and the photogs have a custom of waiting on public property to get their shot. That makes sense, because the code to the cage presumably keeps non-members out.