> Some have A records pointing to the root servers. I’m not sure why this is - I think it might be because no DNS has been configured in the TLD zone for these TLDs.

I was also curious about this. It turns out that it's because dig interprets some TLDs as DNS record types (MD, MG, MR, MX) or DNS classes (IN and CH) of the same name! Not sure why, but when given only a type or class (without a domain name), dig actually queries for a list of the root servers, which explains the strange results. Looks like using the -q option to specify the domain name works around this.