It's not as sudden as that.

For reference, I admin a mail server for a small charitable organisation, and our mail graph for the last year looks like this:

http://dl.dropbox.com/u/4957647/mailstats/mail.png

http://dl.dropbox.com/u/4957647/mailstats/rejected.png

Where the grey is spam filtered by spamassassin and the red is obviously spam rejected outright due to an invalid recipient or an impossible sender or whatever.

The previous year's trend looked sort of similar - I think 2007 or 2008 was the peak in terms of spam we receive and it's been declining on average since then.

...and tech-savvy people visiting their relatives forced to do some tech support.

I guess that contributed plus a spike in people starting to use iPads and smart devices and Macs and Kindles etc and either abandoning or just not having their Windows desktops turned on as much. Botnet agents can't send spam when turned off or offline.