The first time I heard of someone getting DFd, the offense was included in the announcement.
Later, another young person was announced as DFd "for conduct unbecoming a Christian" - which soon became apparent when she started wearing maternity clothes to the KH.
The first time I recall hearing an announcement regarding disassociation, it was not even someone from my congregation, but an unbaptized minor who had done some volunteer work for the army cadets, again "conduct unbecoming a Christian".
The announcement changed in the mid-80s, if I remember correctly, following the Ray Franz fiasco at Bethel. It seems to me that the WTS was becoming more concerned about what was going on between a person's ears in terms of what they believed rather than just "conduct". Ray Franz's conduct was still in keeping with the moral principles that he learned as a JW - it was his beliefs that evolved. That is also the reasoning behind the change in the baptismal questions, which currently include a kind of pledge of allegiance to the WTS, and acknowledges the WTS (aka "God's Spirit Directed Organization") as the source of salvation rather than God or Jesus.
Now they just announce that So-and-so is "no longer one of Jehovah's Witnesses". You don't know if they are DFd or DAd. It ramps up the level of paranoia, and it basically gives no quarter to someone who has conscientious objections to WTS policies. I fully anticipate that the literature will soon make no distinction between disfellowshipping and disassociation - when you knowingly engage in conduct that is contrary to WTS beliefs, you will be regarded as having disassociated yourself by your actions. This will effectively do away with judicial committees - and JWs will just need to make a report to the local elders and have the action corroborated by another person or a couple of elders going on a fishing expedition, to make the disassociation a fait accomplit.