The server restarts due to a number of things.
Firstly, because there's literally tens of thousands of mods, addons, plugins etc installed on the server, with 20 people playing at once, the server needs to download those file to EACH individual player to make sure you all get the same gaming experience, Sometimes the server can get confused and not download certain packages or download too much and crash itself.
Secondly, Buggy script, we're also aware that somewhere in the lines of code in WCS there's a few bugs and incompatabilities which cause the server to crash/reboot itself in order to try and repair the errors, unfortunately, as the code is hand-written by myself, mentoz, pooky and a few others the server isn't allowed to change anything, so it becomes a crash instead.
There's not alot we can do at the moment apart from remove addons (disable) one by one and figure out which one stops the crashes when it is disabled.
Im sure there's more reasons out there too ¬¬
Most importantly:
We do apologise for the crashes and we are sorry if they annoy you, this is just what happens with WCS on CS:S.
All of your XP, Races and anything else you have, is SAVED so if the server crashes as your selecting skills, don't worry!
we have had reports of people losing a level during a crash and what we ask is when the server crashes, take a screenshot of your current level (type playerinfo, select your name) then paste it here, and we will reinstate however many levels you have lost.
The reason we ask you do this is because, we have had numerous people say "ive lost 5 levels!, I lost 2 levels!" as you can see, it's a cheap way to get free levels and we don't tolerate liars.
Anyway,
Hope that answers your questions!
Moo.