Skype relies upon peer-to-peer networking to create a large message-passing network. So your computer (if you have Skype installed) is passing Skype traffic when it's turned on. When Patch Tuesday happened, several dozen computers were rebooted, and if their Skype is not set to auto-logon, the computer doesn't log on to Skype and all of a sudden this peer-to-peer network disappears.
There was a second part of the problem. Skype had a previously unknown bug on their back-end servers, and when their peer mesh network got crippled, this bug manifested and kept them down longer than was reasonable.
http://blog.wired.com/gadgets/2007/08/microsofts-patc.html