Your last experience in Feb 2010 does seem a bit over-the-top strange. Perhaps there was some special tip-off on that day or for that flight, or maybe they were just doing some sort of super-drill. I suspect there might have been some sort of US official lurking somewhere around to do a "spot inspection" of procedures, and you just got unlucky with your timing.

Usually in addition to the standard document check and security screening that all passengers go through at Chinese airports, there will be an additional manual secondary check of all carry-on bags and often of the passenger (hand wands, pats), somewhere near the gate. This occurs for:
1) Flights on US airlines regardless of next destination (i.e. NRT); and
2) Flights bound for the USA on any airline.

Not only at Shanghai, but this is fairly standard for other Chinese and other Asian airports, for outbound USA flights and/or on US airlines.