Along with most of the media, expert commentators and the US administration, I wasn’t expecting that. The best explanation I’ve seen for what happened is this –

Most likely, Mubarak interpreted what he planned to do in his own mind as a kind of stepping down, and gave sufficiently ambiguous indications that those around him misunderstood his intentions (Juan Cole).

Alternatively, if you can cope with humour, this – from Stalin’s Moustache – is pretty good.