Programs vary in how and when they decide to offer interviews. Some programs will offer applicants, who appear to be strong on paper, an interview offer early before their application is complete (e.g. Board scores, all LORs and the MSPE have been received) while others will wait until later (after Nov 1) and offer interviews in mass even to the stronger applicants. Weaker applicants might not receive offers until after Nov 1. Programs tend to not send rejection notices to applicants.
Here is the statement from the residency director's group:
http://www.aadprt.org/documents/AADPRT_Resident_Applicant_Guidelines_part_1.pdf
If you think you are a strong applicant and have not received interview offers from places that are already extending offers, first check your spam filter. Every year there are 3-5 applicants whose interview offer never reached their inbox. If that is negative, go ahead and call the program to see if a decision has been made.