>traffic changes to one single cause
When they tie to a specific event at a specific time - they are quite easy to locate. No change in referrals during the so called 'may update' (we have seen zero change - as far as I can tell - there wasn't an update), and then stuff happens the morning they roll out the new layout. 1 to 1 correlation = causation.
> "Page previews"
yep. But how is that any different than 'cached' page.