What are the reasons why a payment submission through paypal would be pending?

A payment submission may be considered as pending when a user closes the browser or doesn't complete the payment after being redirected to the Paypal page. As such, their submission will be treated as an Incomplete Payment.

Please note that completing a pending submission will NOT process any payment, as there was no actual payment from the start. The system will simply allow you to treat it as a regular submission for the records, and also allow to send the email alerts as usual.