Page ranking plummeted in google - still great in Bing

[kirklott]

On one page on a client website (this one: http://www.diversityresources.com/diversity-calendar.htm ), in the past couple months, the ranking for our keyword phrases plummeted - one went from #16 to #81. 

I can think of only 2 reasons:
a)   The client made a number of changes – including removing optimized headlines – without my involvement. He harmed the optimization of the page.
b)   I might have clicked on the page URL from within a link building subscription service - google would know that someone went to that page from within the link building subscription, and punished me accordingly.

Interestingly, we are at #1, #2 and #3 for our top keyword phrases in Bing, compared with #26, #61 and #81 in google.

Your thoughts? Why did this ranking drop so much?