It's probably connected with the backup scan before the backup itself starts. Since it doesn't actually use the backup drive Backup Exec can do more than one server at a time, and then once the scan has completed it will put the job into the queue if there isn't a drive available. If the second job takes 15 minutes to do the scan, and the third job only takes 5 minutes (eg the second server has a lot more data, or a slower connection), then the third job will get put in the queue earlier than the second, and will be started as soon as the first job has completed.
So yes if you increase the amount of time between the jobs starting such that they can all finish the scan before the next one then you should be fine.