Recently I started to notice I very high number of LRU Nuked Objects on my websites, which was essentially wipping the entire Varnish Cache. I run Varnish with 4GB File cache and site ocntent is mostly served by external "Poor Man CDN". So, in theory my site content should not be anything near 4GB, however, Varnish Cache was running out of memory and "Nuking" cached objects.
Sizing your cache
Watch the n_lru_nuked counter with varnishstat or some other tool. If you have a lot of LRU activity then your cache is evicting objects due to space constraints and you should consider increasing the size of the cache.