I think some people have decided that making a net 50 cents or less per day per card while driving up heat/noise/electric costs and driving down the longevity of their cards, isn't worth it, especially as BTC prices have gradually dropped.
Also, the slowdown in hashing power has not been sudden; it's been gradual. I think people have gotten their first "mining" electricity bills and the sticker shock got to them. Hardware is only part of the expense; the ongoing electricity cost can get expensive. In the US, many states have "tiered" rates in order to promote energy conservation/efficiency, so mining can easily bump you up a couple of brackets into crazy-rates territory. I'm paying an extra $60-70/month just by running two mid-range cards 24/7, compared to my pre-mining electric bills going all the way back to last year (so it's not just seasonal fluctuations).