I'm pretty clueless on it as well. It sounds like the emails where being queued on the SMTP server for some reason, and something happened to trigger the SMTP server to send them.
Here is an over view of how emails are sent on order success:
The to email address is gathered from a session variable, the PHP code compiles the email message with the cart information and configures the email.
this email message is then sent to the SMTP server.
The SMTP server relays the email message to the to address.
once the php page is done loading, the message has been transferred to the smtp server.
In the case of PayPal Payments standard, an IPN post is made to your site from the paypal server to trigger the email. I suppose its possible that the IPN post was delayed for some reason if you are using Paypal Standard.
