Thank you for your responses. I was able to receive help from an employee over the phone which was great. I will keep your replies in mind if I need it in the future.
For anyone else seeking the answer to this, what had happened to me is that we had previously had an active trigger campaign that was syncing all uploaded leads into Salesforce. The gentleman on the phone helped me identify which campaign this was and how to exclude my send.
All I needed to do was create a filter in that Smart List which excluded the list I was about to upload. So you'll need to create the list name, insert the filter in Smart List for the trigger campaign, Create a program to affiliate the list with and then upload the leads.