Google make micro adjustments to their search algorithm on an almost daily basis. There are sometimes bigger updates which occur perhaps once per month, or every other month. Lots of website owners do not realise that these little changes can mean massives changes in the rankings!
The different algo updates have all focussed on different areas of websites, so they could relate to content, relevancy, links or onsite optimisation. There are often no clues as to what the updates have included, as the Google staff are always very secretive about their actions. Often, there is no real confimation of an update at all, with many website owners just seeing that their keyword rankings have taken a nose dive.