Domain Authority is used to track the the “strength” of your website compared to other websites. According to Moz, this is metric is calculated "by combining all of our other link metrics—linking root domains, number of total links, MozRank, MozTrust, etc.—into a single score."
Juggle Cats has 13 Root Domains and 41,022 Total Links. Most of which seem to be coming from sites owned by the same parent company called Proquantum, a _"Special-Interest Directory Website Publishers since 2002" _ In addition to Juggling Cats, they also own Engineers Guide USA, Health Guide USA, AssessorLinks, and Doomsday Guide. Each of these websites includes a footer that links to all of the other sites (and this footer can be seen on every page).
That being said, a domain authority of 30 for a site that's been around since 2002 is nothing to write home about. I've worked with companies who have newer sites that look terrible & aren't even optimized for search, and they'll often still have a domain authority between 30-40. This is because domain authority is more about #links than anything else, and more specifically it's about the # of linking root domains. In this case, it seems like the huge # of total links might be pulling up the overall domain authority slightly...but this isn't really significant in my opinion.