Many dns queries to a.root-servers.net

Hi, I'm seeing quite a lot of queries for "a.root-servers.net IN A" in the logs of my caching servers. They seem to be coming from home normal DSL customers (IPs who would be expected to be using the name servers) with each sending one query every 2 seconds. They all together represents more than de 10% of the total queries. I am guessing it is probably some sort of spyware/malware/virus/router/O.S. version but I was wondering if anyone knows offhand?