Forum Moderators: buckworks
I find that serious spikes tend to be press coverage, and it can be very difficult to know where it is all coming from.
You could try doing a search on G for pages that contain the term 'yourdomain' then if it is a big media player with a website you may find it.
Another option is to see which pages are enjoying the spike the most, this can also help you track it down.
If it is accross the board then perhaps the site is just getting busier but if it is to specific page(s) then perhaps there is an article out there about X and your site has been listed. Or perhaps your site features highly in the SERPS for a hot topic.
What I do is to use spikes (one off spikes) as a target. So for example we had a serious spike aroud one year ago, this enormous line in the stats on an already busy site. I then set a goal of achieving that level of traffic within the year. Now I am waiting for the next spike!
Perhaps it could be somethig random like that, how long did the spike last?