"The Eagles ended on a rather abrupt note, although in retrospect I realize now that it had been ending for quite some time"

This quote by Don Henley is most likely referring to the band The Eagles and their eventual breakup. The Eagles were a popular rock band in the 1970s and 80s, and their separation was sudden and unforeseen. Henley is likely reviewing the separation and how it felt at the time. He acknowledges that the end of the band had actually been coming for some time, however it still felt abrupt when it took place. He likely recognizes that the band had actually been dealing with internal problems for some time, however he was still surprised when the band lastly ended. Henley's quote is a reflection on the suddenness of the break up and how it felt in the moment, however likewise a recognition that completion had been coming for some time.