Can anyone explain this?

[Jonnygeeuk]

On Sunday 26th May, for about 40 minutes, we had about 25-30 direct visits from San Jose (we are a UK site). During this time our rankings increased dramatically and then as soon as the direct visits disappeared, our rankings went back to how they were prior to them visiting the site.

[TakeshiYoung]

Google results fluctuate all the time. It could be that Google was testing something at the time which caused your site to rank highly in San Jose, leading to increased traffic.

[AlanMosley]

It could be that something went viral that caused your site to rank higher for a limited time.