According to Urban Dictionary, a Dog Pile is:
A group of people jumping on on person and creating a tower of people while crushing the people on bottom.
This term is used in the area of cache invalidation too. It means that when a cached object expires, it causes a rush to refresh the object from source, which can sometimes lead to the source being overwhelmed with requests.
NginX has protection against this when using its reverse proxy cache feature. More in the wiki.