I and others have commented on this numerous times.
At one time, many years ago, everyone clapped.
The whole thing changed in the 1980's, early 1990's (?) or so when there seemed to be a lot of people who would commit adultery with someone else in the congregation, get DF'd, remarry, then get reinstated a year or so later.
The "innocent" person(s) may still be in the congregation when the reinstatement was read, people clapped and later everyone went over to hug and kiss the ones reinstated.
Obviously, no matter how you look at things, not a happy situation. The order from the top was to stop the clapping in respect of the innocent party(s).