what causes incomplete submission

If you're referring to Incomplete Payments, these are triggered when the respondent/user failed to make a payment. It can be caused by a plethora of reasons (usually, if they just ignore the Paypal Payment page after form submission then that would be an Incomplete Payment).

In short, if they submitted the form but never paid, then it's logged as an Incomplete Payment.

What are Incomplete Payments?

Each submission on a Paypal-integrated form is initially considered an Incomplete Payment entry. JotForm uses the IPN message to determine whether a submission's payment is successful or not. If successful, the submission is marked as completed -- the post-submission actions are then performed -- such as sending out the notifier and autoresponder emails and sending the submission data to WebHook URLs (if there is any). 

If the payment is not successful, the submission remains Incomplete.

Complete guide: Paypal-Incomplete-Payments-FAQ