Found an interesting article here:
https://en.opensuse.org/SDB:Disk_space
It says:
“Old kernels
On Tumbleweed old kernels can build up.”
No further explanation why this happens.
It seems to be random…
Any idea why?
Hi
Not seen it here… purge kernel systemd service works fine and respects the zypper config settings (running+1).
The link is an official opensuse wiki.
Strange…
Maybe it was a bug, fixed by now.
In that case the opensuse wiki could be updateted 
There was a period, perhaps around July 2020 (but my memory might be off) when the purge-kernels service was not working. At that time, kernels tended to build up. That problem has been fixed.