Most likely low memory, linux systems when there are out of memory calls the OOM killer, which kills the largest processes running on the system. https://unix.stackexchange.com/questions/153585/how-does-the-oom-killer-decide-which-process-to-kill-first
You can probably find evidence of this in the system logs, on most modern systems, this should show you higher prio logs from last boot:
journalctl -p 4 -b
You should see something like this, if it was killed by OOM killer
MESSAGE=Killed process 3029 (Web Content) total-vm:10206696kB, anon-rss:6584572kB, file-rss:0kB, shm em-rss:8732kB
And you should definitely think about extending the memory of the system or reducing the number of containers/they memory footprint.
Why do you really need this ?
Why not just set up a VPS and deploy the NginxProxyManager to it together with a wireguard tunnel to your home system.
You really dont need 2 proxies...
Or if you want to keep the NPM localy on your home server, then you just setup wireguard on VPS with NAT and port forward to your tunnel.