There was talk about a tax on M&A activity
I'm no scholar on the 1987 crash but I've heard the usual explanations. One that I read this week that makes sense is from Matt Maley. He talks about the government had floated a trial balloon regarding a takeover tax.
He remembers that triggering worries that were already in the market. When it wasn't withdrawn, the selling accelerated and portfolio insurance sales kicked off.
Here are some news reports from that day.