I've noticed an unusual high usage of my upload, whenever a download is running.

The upload is usually somewhere 5-10% of the download bandwidth usage.

I assume the VPN connection itself requires the upload for something, but I couldn't figure out what for.

OpenVPN 2.3.7 on Debian and the VPN tunnel uses UDP.

**edit** No torrents. 
e.g. If I download Kali Linux via the official host, I currently get 180-250 kB/s, the upload is ~50kB/s..

I've had cases where a HTTP/S download would get ~4MB/s and the upload maxed out my bandwidth with ~380kB/s..