All photos deleted from iPhoto and Time Machine

Recently I opened iPhoto only to find that all my photos (apart from about 20) had disappeared, along with slideshows and albums. Looking in the iPhoto library, I found that it had been emptied. Nothing in the Trash. So then I went to Time Machine - no trace of the missing photos in Time Machine, despite the rest of my data being there.

Fortunately, I had cloud backup (Zipcloud) and I was able to restore my photos/albums/slideshow.

But what happened? I hadn't gone near the iPhoto library (I never do), nor done anything unusual on my Mac. The only thing out of the ordinary was that I accidentally unplugged my external hard drive from its power source just before I noticed this problem.

Reading Communities revealed that others have had this problem and there is a theory that running Time Machine while iPhoto is open can lead to problems.

So my questions are - Is there a bug that causes iPhoto's library to be deleted and why was the backup library gone from Time Machine?