On a shared environment the 'Mail Queue Manager processes' e-mails in order of up to about 500 queued e-mails. However, when it goes above this amount then the system switches mode and starts processing emails alphabetically (this is much quicker)!
If this is happening then it is possible that an email next in line could end up at the back of the queue which could cause a delay. Also if the e-mail was rejected on the first try for some reason the server will queue it and reattempt to send it later for you.
It is unusual to be over the 500 queued mark however, if several accounts on your shared hosting environment are sending out mail-shots around the same time, then it is conceivable that this could cause a backlog.
If this is becoming a continued problem then you are more than welcome to raise a ticket and we will be happy to check it over for you.