Some schools will choose to release invitations steadily over the course of the season, with invitations issued a few weeks before the proposed dates, with the exception of the period around the AAMC annual meeting (Nov 3-7) when many med school leaders will be out of town and unavailable to conduct interviews, as well asThanksgiving week, and the week between Christmas and New Year.
Some schools have found that booking interviews too far in advance leads to applicants making requests to reschedule as things come up. So, they are going to pace themselves and offer dates a few weeks in the future. You might be queued for interview but the invitation won't go out for weeks when the dates available are a bit closer.
Rejoice that you have some interviews already and be patient with regard to the rest.