Them uploading at same time is due to Partial File Sharing, which is good for the network because it speeds up everyone's downloads. ie: many more sources.

However speeds usually drop near the end and you will find less sources also.

Download % regressing will either be due to a (i) some file hashing problems as it detects corruption & tries to re-download the section where the corruption is detected, else (ii) LW bug, else (iii) corrupt or fakish file.

About a week back I had a video redownload back to perhaps 26% many times. It would reach 45 or 60% then go backward. Eventually it did complete. I did an experiment. I went to the incomplete folder & opened that file & found it was complete anyway. So ... I made a copy and moved it. But continued downloading the other. Both were the same in the end.

I think this has to do with popup windows. If you set the corrupt file message to always give a particular answer. If yours was always continue download then that it will.