piatkow - 11:00 am on Oct 30, 2013 (gmt 0)
Basically either the ads have changed or the visitors have changed. Most likely either the good inventory isn't there at the moment or Adsense's algorithm is mistakenly putting in less suitable ads based on the wrong keywords.
I have had CTR fade to almost nothing and then come back more times than I can count. I know from watching my site over a period of years that the best CTR is at times when I get ads for events such as concerts or festivals and these aren't consistent across the year.
The last couple of days have been poor for me and looking at my site now I see off topic ads including one so far out that I am about to block it.
I am assuming of course that you are talking about a drop in the total number of clicks rather than a surge in visitors from a new source who are not clicking and distorting the percentages without affecting cash income.