Not necessarily the case. The traffic for our high ranking keywords has diminished despite their rankings going up.
Right - we're not talking about overall traffic, but how many people found us for "top keyphrase" from one day to the next. I've seen the traffic decrease with higher rankings, and also increase with lower ones. Very strange.
Is Google losing market share to something else?
In my niche, definitely, but this would NOT explain why I saw an increase in traffic when my rankings lowered slightly. I think it HAS to be something Google's doing - some kind of testing or something.