Here are the stats on my sites that dropped.
8 sites went from page 1 to page 10+ (some are not even in the top 1000).
These sites were all on the same shared Hostgator hosting.
I have at least one other site on that same shared hosting plan that is still on page 1.
All content is handwritten, nothing is spun, it is all high quality content and the average on page time is measured in minutes (because people are reading the high quality content).
I didn't do any linkbuilding that involved automation, spam, or spun content.
Backlinks are varied. I don't think overoptimization is the issue.
I noticed the sites had dropped this morning when my AdSense impressions were way down.
I don't think it has anything to do with spammy backlinks because I never did any of that on any of my sites. IMO, spammy backlinks were devalued months ago anyway
I also don't think it has anything to do with hosting, as I have at least 1 site on the same hosting plan on the same IP that is still ranking well
Have a look at what fad3r said:
I have 4 sites all using the exact same tools, linking strategies, etc 3 out of 4 are fine. 1 vanished for 4 out of 5 keywords so I cant imagine that is the case.
Same here. I pretty much do the same SEO for all my sites. There doesn't seem to be consistency in what is causing this drop.