Do you remember which gate it was in DEN?

All of the gates (that have envelopes big enough i.e. B46,B44,B42,B39, B45, etc. but not B15 or B16) are set up to accomodate the 757 from door 2. If the bridge was on door 1, then that means that the jetbridge CSR made a mistake, or that the plane was poorly parked. It is most likely the former.

Door 1 is an unsafe door to use for the reasons that fastair explained. It is a difficult door to open and close (employees have a higher risk of injury), it is smaller, and the boarding takes longer.

Not a practice that is supposed to take place. That aircraft is hard enough to board on time through door 2!