This can happen when the account already had an SG pledge with the same sustaining_gift_number. TA sets the schedule_start_date on an SG pledge by looking for a previous one first. It first tries to calculate the schedule_start_date using the date_last_pulled of the previous pledge. If there's no date_last_pulled, it will try to use date_last_billed, and failing that try schedule_end_date.
In this case, the previous pledge had no payments on it, so there was no date_last_pulled or date_last_billed, so the schedule_end_date was used. It set the new schedule_start_date to the schedule_end_date plus the number of days in the frequency of the pledge. So with a frequency of 'M' for monthly, the schedule_start_date was set to the next month, 30 days later.