According to the below article, in the late 80s, De La opened for Bob Dylan in Amherst, MA (Pretty sure the date is wrong and it's referring to the show you speak of, as I have trouble believing DLS opened for Dylan as early as '89). Anyway, the article suggests the reason they were a no show was because they had some, ahem, car troubles along the way. It's possible this story isn't true; if it was, I guess you would have known at the time:

http://every-day-i-write-the-blog.blogspot.com/2011/05/bob-d...