Jack sighed. "Think, Ianto. You've got a good brain, use it! Human psychology, you're transiting from one environment to another, the door is the most common and widely recognised portal - so whatever it is, your mind is going to present it as a door. It's how it can project into your dimension - it's got to obey the rules of your reality." Whatever it was.
He ignored the others. For those who didn't come through a door, he was willing to bet that it was a familiar portal. TARDIS or otherwise.