I told my grandfather to invest in AMD a few years back when they were at $1.9/share. A month or two ago they were at $15/share.. Why did he not listen to me?!?!
To answer your question though, there is no good answer for this. It's difficult to pinpoint the behavior of a stock to a single event, unless it's something so absolutely obvious, but in this case, AMD's stock behavior is most likely due to various analysts undervaluing the stock, and shareholders acting off of those assessments.
There doesn't seem to be anything so apparent as to why this drop occurred, other than people don't see the stock value getting much higher before it gets much lower. IMO, AMD stock will hold under $15/share for a long time to come, and those who sold were smart in doing so (assuming they got in at a profitable time).
Cheers.