I also noticed this behaviour and I thought initially it was caused by my specific setup.
So it is not probably... I noticed that after removing an user who has also a addon domain or subdomain
apache actually sometimes stops.
. The cause is as far as I know the remains of subdomain / add on
domain in apache config..
I use a very simple setup all domains of users on the server are a subdomain of the hostname. Maybe it
is because this setup but is annoying as hell because apache / the sites do not respond. Webserver is
down after removing.