Argiope Behavior

It isn't clear to me why Argiope (and other orbweavers) react by shaking their webs. One possibility is that this is a defensive reaction, but when truly panicked they typically drop from the web entirely. I've seen them react this way to prey, and the movement sometimes assists in ensnaring the prey in the web. If that were the case the spider would rush to the offending object, and if it was "deemed suitable" the spider would wrap the prey. It is also possible that this behavior assists the spider in determining exactly where in the web the prey is. The prey often reacts my moving, and this is helps the spider locate the prey in the web.

I have observed a number of orb-weavers to use a similar behavior when threatened. They vibrate their entire webs, disappearing temporarily as a blur in the center. The spiders I have observed don't do this in response to a potential prey item, though, only when threatened. It does really sound like your Argiope thought she had a prey item, though. This is very interesting, and I'd love to hear what you come up with.