There have been enormous number of discussion about this issue, but unless Google gives more informative answer to our speculations I am posting my comment on that.
I have looked into the pagesyndication .js code and it is very possible that they are storing referrals to pages that show ads.
The traffic may be coming from a few sources:
1. Direct entry of the url into address bar - no referrer - using bookmars or a well known brand name url.
There is no problem with big brands, but small sites do not get a lot of traffic in that way. So if a lot of clicks are generated without referrals that gives that a light to more closely look at the website.
2. Pay Per Click - low bids on high paying keywords - very tricky but possible - this traffic is quality traffic & click originating from that source should be counted as valid. They do not look closely at this source of traffic.
3. Traffic from other sites - this traffic is looked more closely into - of the referring site is using some spammy techniques to direct traffic to the target site this may be easily detectable looking at those sites. If those sites also encourage you to click on the sponsors on the next page that is very easy to detect. This was in a case that Jenster found as a reason for one of the accounts' termination.
4. Traffic from SE - this is a legitimate source - but according to some members if the traffic from SE get's too high they remove the site from the index leaving AdSense account. It depends on many other factors but it can happen.
This is only my two cents to the AdSense cancelation mysthery.