I had a stray cat that could open doors by putting paws on both side of the door knob and turning it. It would then claw under the door to open it all the way.
It would attempt to open the front door but couldn't because that had a release latch built into the door handle on the inside. Could always know when he wanted out by seeing him try to open that door.
I chalk it up to a survival instinct.
We have a cat who taught himself how to open the bedroom door. My wife didn't believe me until she saw it herself. (He'd climb up on a bureau in the livingroom and then work the doorknob until the door swung open, then he'd go marching in, and jump up on the bed.)