The Portable job cannot be run simultaneously with any other backup jobs, so it must wait in the queue until the Standard job is done; thus, you can see how a Portable can continue waiting in the queue if scheduled Standard backup jobs keep hitting the queue before it gets a chance to run.
This also applies to Tape Backup (TAR) jobs. Their queuing is handled the same as a Portable backup job.
Typically, Standard backup jobs are a priority to most organizations over the Portable jobs; however, if you want to have your portable jobs run on schedule, disable the setting, Run Queued Standard Backup Jobs First. This is found in the Reload administration console: SYSTEM | JOB | PREFERENCE. It is a toggle setting. Pressing ENTER when it is “enabled” will disable it and vice versa.
For suggestions on how to speed up your portable jobs, refer to KB article entitled, “How to Speed Up Portable (Full) Backup Jobs”.