There's not enough information here to properly answer your question. "Upper rear door" doesn't make sense to me. I wonder if the door in question leads directly outside. I wonder if this door leads to a common area or into a private suite.
Regardless of any of those issues, the other tenants in the building don't have any right to pad lock common doors or doors that lead from a suite that isn't theirs into any part of the building other than their own unit. So, if the door doesn't lead into their own unit, then tenants have no right to pad lock the door. If the landlord pad locked the door, then he/she should have told you that this change was being made. If the landlord didn't inform you of this change, then you have an issue with the landlord: is the door required for safety/fire code? Is access to this door addressed in your lease? In some cases, since you have been using the door for some time now, that door has become a part of your leased premises just by the act of your using it and the door not being locked. That can change of course if access to that door is not properly addressed in your lease, but in order for it to change, the landlord will need to serve you written notice. If you have any further questions, you should contact a RE attorney in your area (or on LinkedIn).
May 10, 2013