Question about Clinton’s Approval

I was exchanging messages today with Charles Franklin, who was a key person behind pollster.com and is responsible for both the Marquette Poll and the Survey Monkey Poll

We got talking about Clinton’s approval ratings.  Clinton is really the only other President whose approval ratings tanked in the first 6 months.  Does anyone remember why these numbers changes so fast in May?

 photo clintonapproval_zpszxetskyg.gif

Here is the file with approval ratings for every President since Carter.  Warning – right now it is raw:
https://www.dropbox.com/s/zsd7w31iyv7ukz6/gdp.xlsx?dl=0