It is an appositive phrase. An appositive is considering pronoun or noun that can identify other noun or pronouns. In this case, ballroom dancing and the tango is identifying ''performance art'' in this sentence.
This kind of phrase can show us the information about the noun or pronoun which is causing a better understanding of the sentence.
Without the appositive phrase, many sentences would be without a sense or they can have various meanings.
Maybe it's because not everyone can cope with each other after such a long time. Maybe not everyone has great social skills after living together for awhile.
