Instant link?
Given that Quality score is based on how long Google users stay on destination sites and how likely they are to click on sites in the results, it might seem that Google Instant and/ or Instant Preview are messing things up; either through Instant Preview pulling realtime pages through and/or Google Instant serving up masses of irrelevant results as a consumer types in a search phrase.
Either way, the timing is... coincidental, at best.