What do you use as a torrenting client? Most popular ones give you the ability to choose a specific interface over which it will allow incoming/outgoing connections to other peers. Your ProtonVPN should have its own interface you can select from your client. That should make it much less likely for that to happen again if Proton crashes, since if Proton crashes, that network interface disconnects.
The NIC thing was more for if you were using a VPN. You can lock down your client to just use the virtual NIC your VPN client creates, so that’s always recommended when setting up your client.
What is your toreenting “signal chain”, so to say? Normally when you download things through qBittorrent, are you generally running bare? Do you use a VPN? Is your torrent client configured to use a specific NIC? If so, is that NIC active and passing traffic? There are so many variables that play into this.