At that time('89) there was speculation that Robert dropped out of the Who performance because Townshend made an unflattering comment about Zeppelin in a Who cover story in Rolling Stone. I'll try to find the direct quote but I believe the genre of Heavy Metal came up in the interview and Townshend, to paraphrase, said it became "that gross, disgusting object that was Led Zeppelin." Shortly after that interview was published (June 89'?) Robert dropped out of the Tommy performance. Rolling Stone even commented on Robert's dropping out a few issues later in their Random Notes section that it may have been due to Townshends comments. I'll try to post the original quote because I just found that issue in a box of my old magazines a week ago.