If there is no condition, it will be activated.
The second page is conditioned by self switch B. When self switch B is on, the second page will be active. When self switch B is off, the first page will be active because it has no conditions. Think about what you originally had ... page 1 with the autorun trigger. That had no conditions. How did THAT page get activated? It's not the trigger - the page has to be active before it even checks the trigger. It was active because it had no conditions. The trigger just made it run automatically rather than waiting for the player to interact with it.
None of this has anything to do with a door being open or closed. That's another thing you left out.
In that case, don't use self switch B - use a real switch instead. Wherever I've used self switch B in my images, use the other switch instead (so turn on, turn off, and condition on second page).
Then you can have two pages on the door event. The first page will be a closed door. The second page will be the opened door with the transfer command, and will be conditioned by the same switch.
Not sure how you're going to work this if the player DOES get through the door in time though - they'll be on another map then, and the parallel process checking for the timer won't run anymore. Maybe immediately before or after the transfer command, also add commands to stop the timer and turn the switch off, so it'll happen as you leave the room.