Sometimes varnish cache an index of (?C=N;O=D)

I have a issue with one server, this server hosts an wordpress with wp-super-cache, apache 2.2 and varnish 3.

Sometimes the varnish caches an blank page or index of page with this query “/?C=N;O=D”, When it happens the number of real time visitors really drops, like the image below:

visitors drop image

Sometimes the user number drops from 700 to 50 very fast.

The VLC that I use is it: https://gist.github.com/diegoos/f1411ca56d061ed5ae25

Does anyone know what can it be?

Thanks.