Orders Automatically Switching from Processing to Pending

I am using wordpress with woocommerce. When we receive an order we print out the invoice and mark it as processing until we get the order together. This allows the customer to get a notification that we received the order and we are working on it.

Some of the orders that we mark as processing are turning back to pending, as if we did not see them. There doesn’t seem to be any pattern to which ones revert back to the pending state.

Anyone have any idea what that is about and how to fix it?

Thanks!